Romans 2:4,5
Or do you despise the riches of his goodness, forbearance, and patience, not knowing that the goodness of God leads you to repentance?  But according to your hardness and unrepentant heart you are treasuring up for yourself wrath in the day of wrath, revelation, and of the righteous judgment of God;

God's forgiveness is primarily contingent upon repentance. But his forgiveness is absolute. "As far as the east is from the west, so far has he removed our transgressions from us." Ps 103:12 And "I will remember their sins and their iniquities no more." Heb 10:17

"Repent therefore, and turn again, that your sins may be blotted out" Acts 3:19

Repentance of itself is not simply a reform of one's behavior, although genuine repentance does result in a reformed behavior. But repentance itself is the sincere seeking to be forgiven. It is preceded by conviction and acknowledgement that one has sinned. The kind of repentance God demands is one which has visible results. Paul discussed his gospel with King Agrippa saying "that they should repent and turn to God, doing works worthy of repentance." Acts 26:20 And also John the Baptist says, "Bring forth therefore fruits worthy of repentance."  Luke 3:8

So also for us, if indeed we forgive as Christ, there are times when we should grant forgiveness of sins done against us if we recognize they are so blinded by their sinfulness to what they are doing as to be devoid of conscience. The examples of Jesus on the cross and Stephen and Acts 7 where such forgiveness was requested contain in them blindness due to religious zeal. While such forgivness is optional, as the Spirit guides, perhaps an application in the present day is forgiving Islamic terrorist who in their religious zeal murder a family member.

Those sins which are committed against us Christians are also an offense to God. In the sheep/goat judgement of Matthew 25 the Lord will bring judgement on such things. But if we forgive others of their sins against us, then the Lord will also. But when it speaks of retaining sins (which means holding back such forgiveness) it implies that doing so is appropriate at times. Though that verse doesn't specify when it is appropriate. That subject is covered in other verses like the following:

Luke 17:3,4
So watch yourselves.
"If your brother sins, rebuke him, and if he repents,
forgive him. If he sins against you seven times in a day,
and seven times comes back to you and says, ’I repent,’ forgive him."

Notice the condition of repentance here.

Matthew 18:
15 If your brother sins against you, go and show him his fault,
just between the two of you. If he listens to you, you have won your brother over.
16  But if he will not listen, take one or two others along,
so that ‘every matter may be established by the testimony of two or three witnesses.’
17  If he refuses to listen to them, tell it to the church;
and if he refuses to listen even to the church,
treat him as you would a pagan or a tax collector.

19  Again, assuredly I tell you,
that if two of you will agree on earth concerning anything that they will ask,
it will be done for them by my Father who is in heaven.  (Xref John 20:23)
20  For where two or three are gathered together in my name,
there I am in the midst of them."

21  Then Peter came and said to him,
"Lord, how often shall my brother sin against me, and I forgive him? Until seven times?"
22  Jesus said to him, "I don’t tell you until seven times, but, until seventy times seven.

Notice here that if we just read verses 21 and 22 we may get the impression that Jesus is speaking of unconditional forgiveness. But if we read it in context starting from verse 15 we won't have such a misconception.

Notice also the relationship between verses 19 and 20 and John 20:23 concerning forgiving and retaining sin. As Christians when we forgive those who sin against us, the Lord also forgives them for those particular sins. But for those which we refuse to forgive, the Lord will also not forgive those who offend us of these.

Colossians 3:13
"Forgive as the Lord forgave you."

The Lord's forgiveness is not generally unconditional. It is generally contingent upon repentance.

We also have the apostle Paul's example of disciplining a sinning Christian of 1Cor 5,6. He is forgiven in 2Cor 2:7 only after he repents.

But practically how can we determine whether such repentance is sincere? If there's no obvious reason to doubt it, then sincerity should be assumed of course. Forgiveness is granted contingent upon sincere repentance. But if the person's later actions prove that they had not been sincere in their claim of repentance, but rather were trying deceive, then they should be treated as if not having been forgiven in the first place.

But does this mean that God may take back forgiveness he granted to us for sins committing against Him? No it doesn't. Because he knows whether our repentance has been sincere. He doesn't have to determine that by our future actions for he already knows our inner attitudes. And if we had such knowledge, our forgiveness of others would also never have to be taken back. But we don't have such ability. However it is apparent that some are quite perceptive and adept at judging the motivations of others - those particularly with the gift of the discerning of spirits. But then again too many think they're experts at that. So some caution and gravity is called for.

Just as repentance needs to be genuine so does the forgiveness - given of course that the repentance proved genuine. Hatred should not be harbored in one's heart. But does forgive mean to forget? There does appear to be this aspect reflected in God's forgiveness of us. For under the New Covenant it is written, "I will remember their sins and their iniquities no more." Hebrews 10:17 But practically what would this mean? For let's say your friend borrows something of yours and damages it. He repents and you forgive. He then borrows it again and the same happens, and this goes on for some time. Of course you might question whether his repentance is sincere, but let's say it is and what is happening is that he is just naturally clumsy or naturally incompetent or too immature to handle it properly or such. So when he asks to borrow it again, should you forget about his past performance? That doesn't seem to me to be a matter of forgiveness, but rather a matter of wisdom. Though you may not hold any guilt against him just as a mother is not going to hold it against her baby when he wets his pants, yet she certainly isn't going to let her baby drive the car! It's not a matter of guilt, but a matter of wisdom and love. Thus we should forget the guilt of the sins of those we have forgiven, but we should remember the sins themselves for the purpose of edifying that person in areas they are perhaps weak in. But that should be done in love, and not in vengeance, not in the sense of keeping a record of wrongs spoken of in 1Cor 13:5

Some have the misconception that they can live a lifestyle of sin refusing to repent and yet assume they will be saved if they simply forgive others who also live a lifestyle of sin and who refuse to repent. That is not what Jesus is teaching. Nor is his meaning that forgiving others is a work which we must perform in order to be saved. His real meaning is that the children of God behave as their Father. If it is a natural characteristic of our life that we forgive others (in the same manner in which God does), then this indicates that we have been saved and as such all our sins will be forgiven as well.

Mark 11:
25  Whenever you stand praying, forgive, if you have anything against anyone;
so that your Father, who is in heaven, may also forgive you your transgressions.
26  But if you do not forgive,
neither will your Father in heaven forgive your transgressions.

Here again the "anything" is to be understood by way of ellipsis to mean anything from which they have repented, as also with:

Colossians 3:13  bearing with one another, and forgiving each other, if any man has a complaint against any; even as Christ forgave you, so you also do.

Since Christ forgives us contingently upon our repentance, so also it is presumed here that the complaint had to do with a sin which had been repented from, but which had not been properly forgiven. Also in this context it is implied that the "each other" is referring to the Christian community, of which we would naturally expect repentance to be the general practice.

This unforgiveable sin against the Holy Spirit may also be spoken of in Hebrews 6:4-6 and 1John 5:16. which also speak of unforgivable sins. Studies on those sections can be gained through the links.  Although applied even to "Christians", it appears this has to do with the mode of leading one to saving faith in Christ. And thus references to such "Christians" who commit this sin were "Christians" only nominally so on the road to salvation. Nonetheless this is something Christians should be concerned about. For how do you know whether you have been born of God if indeed your behavior indicates otherwise. (2Cor 13:5) The Holy Spirit is what convicts the world of sin, righteousness, and the judgement to come. (John 16:8) And the Holy Spirit often works through other Christians to do so. Rejecting the leading of the Spirit, reckoning him to be an evil spirit, or otherwise rejecting him cuts off access to God. And while Christ provides a bridge between man and God, such a person thusly cut off has no way to find the bridge. One can reject Christ and yet through the conviction of the Holy Spirit be led to Christ later on. But in rejecting the Holy Spirit's leading there is no mechanism in place to restore such leading again. However I believe that the blasphemy or rejection spoken of here is one born of conviction rather than merely an opinion and the blasphemy is not simply a word spoken thoughtlessly. But then again opinions can lead to convictions.

But if we are to forgive as God forgives then is there also a sin which can be committed against our spirit which we should also reckon unforgiveable? If a person demonizes us to the point where they refuse with conviction to repent, and that is their life choice, then it appears they will provide no opportunity for us to forgive them anyway. Thus our mindset should not be whether one has committed an unpardonable sin, but rather whether we are ready to forgive once they repent.

But concerning the most common application of forgiveness of sins of ignorance involves interaction with non-Christians. Paul writes, "I wrote to you in my letter to have no company with sexual sinners;  yet not at all meaning with the sexual sinners of this world, or with the covetous and extortioners, or with idolaters; for then you would have to leave the world.  But as it is, I wrote to you not to associate with anyone who is called a brother who is a sexual sinner, or covetous, or an idolater, or a slanderer, or a drunkard, or an extortioner. Don’t even eat with such a person.  For what have I to do with also judging those who are outside? Don’t you judge those who are within?" 1Cor 5:9-12 Thus we should feel relatively free in forgiving non-Christians who sin against us, regardless of whether they repent. But that is a right and not an obligation. Paul was often abused by non-Christians but only exerted his rights a few times so as to continue his ministry. And in carrying out our individual ministries we also have to tolerate a good deal of abuse from the world, but dealing with those who identifying themselves with "Christ" is another matter.

And so also we have the principle Paul mentioned:  "Follow my example, as I follow the example of Christ." 1Cor 11:1 and thus to behave as Paul behaves in this matter. While he worked toward the salvation of God's enemies, of whom he characterized the former condition of the Christian community (Rom 5:10), he and the other apostles often spoke harshly against those who hardened their hearts against the gospel. For example near the end of his life he said, "Alexander the coppersmith did me much harm. May the Lord repay him according to his works." 2Tim 4:14 But at other times with mercy."My heart’s desire and prayer to God for the Israelites is that they may be saved." Rom 10:1 In fact Paul himself is the ultimate example of one whose sinful behavior God tolerated prior to him becoming a Christian.

Toleration is not the same as forgiveness. For as we noted, there are times where God tolerates people sinfulness, but does not necessarily forgive them. So also in the Christian life. First with respect to non-Christians there are times and circumstances to tolerate their sinful behavior, just as God does, and yet not condone their behavior. There are times to tolerate their sinning against us, and yet not necessarily forgive them just as God doesn't necessarily forgive them.

But there is also another area of the Christian life related to forgiveness, namely toleration or "bearing with one another." bearing with one another, and forgiving each other Col 3:13a Bearing with one another involves tolerating other Christian's sinfulness. So at times we are to deal with other Christian's sinfulness, as for example it says, "I am writing you that you must not associate with anyone who calls himself a brother but is sexually immoral or greedy, an idolater or a slanderer, a drunkard or a swindler. With such a man do not even eat." 1Cor 5:11, and there are times when we are to simply tolerate their misbehavior. Why the difference, and how do we distinguish the two?

The reason for tolerating certain sins among Christians is much the same as tolerating sins of non-Christians - namely to allow them time to grow spiritually and gain insight. And from this principle I would infer what kinds of sins to tolerate - namely sins of which they may not be aware of, in contrast to those which should be obvious, given their particular level of spiritual maturity.

There are many sins committed in the Christian community of which while more mature Christians are aware of, those less mature may not be aware of. There are sins of commission and of omission. For example Samuel confessed,"Moreover, as for me, far be it from me that I should sin against the LORD in ceasing to pray for you" 1Sam 12:23 Many Christians sin by ceasing to pray for their fellow Christians. This is an example of a sin of omission. An example of the commission of sin is 1Cor 8 concerning violating the sensitive conscience of other Christians by doing things that are in violation of the conscience of these other Christians. For example 1Cor 8 speaks of eating meats sacrificed to idols. But there are many applications to this principle. For example it may incorporate the issue of Christian women wearing provocative clothing, Christians who decide to get married after being divorced, or buy lottery tickets, or eat more than they need. And there are many areas that Christians may not be personally convicted about, but their sin is in that they wound the conscience of other Christians who do have convictions in those areas. And thus Paul writes of such behavior, "when you thus sin against the brethren, and wound their weak conscience, you sin against Christ" 1Cor 8:12 Paul's application is to either keep such behaviors, as do not violate your conscience but that of another, private or else just don't do them.

But I would infer that even when such sins are committed in the Christian community, these are examples of what it means to bear with one another. In other words fellowship should be contingent upon each one having a good conscience about their own behavior, and violations of other people's conscience should be tolerated, even though such violations may be sinful. Thus for example while I don't believe in remarriage after divorce, I don't break fellowship with other Christians who do; not over that matter, if in fact they walk in a good conscience and sincere faith.

But just as God's toleration of the sinfulness of non-Christians is not for the purpose of condoning their sin, but for allowing them to repent, so also toleration of the sinfulness of fellow Christians is to allow time and opportunities of spiritual growth so they may gain insight into those areas in which they need maturing. In other words, just because you tolerate their sinfulness doesn't mean you can't talk to them about it.

But even this having been said does not clarify the boundary between what sins we should tolerate and what sins we shouldn't. For example concerning the list in 1Cor 6:9,10 of the characteristic behavior of those who do not inherit the kingdom of God, and which it appears Paul does not tolerate, we have things like  "slander" and "greediness". Yet greediness is often a matter of degree, and slander is often found subtilely in the Christian community either by misrepresenting other people's point of view or casting a slur on one's fellow man. But Paul seems to be speaking of more extreme, lifestyle and obvious practices. Else he couldn't have been categorizing people if these were merely subtle practices. The Bible doesn't really give answers concerning boundary conditions - other than instruction to stay away from the boundary. And thus I can't answer issues of boundary conditions other than to fear God and don't seek the boundary. Afterall, the only ones who need to know the boundary are those who intend to sit on the fence.