Assurance of Salvation

Paul says, "There is now no condemnation for those who are in Christ Jesus, because through Christ Jesus the law of the Spirit of life set me free from the law of sin and death." But Arminian Theology basically says, "There is condemnation for those who are in Christ Jesus, because the law of the Spirit of life has not set me free from the law of sin and death."

2:3 If we keep His commands, then we know that we know Him. 
2:10 He who loves his brother abides in the light, and there is no cause for stumbling in him.
2:17 He who does the will of God abides forever. 
2:23 He who acknowledges the Son has the Father also. 
2:29 Everyone who practices righteousness is born of Him. 
3:3 Everyone who has this hope in him purifies himself just as he is pure. 
3:6 Whoever abides in Him does not sin. 
3:7 He who practices righteousness is righteousness jus as He is righteous.
3:9 Whoever has been born of God does no sin, 
3:14 We know that we have passed from death to life, because we love the brethren. 
3:18,19 Let us love in deed and in truth, by this we know that we are of the truth. 
4:6 He who knows God hears us. 
4:7 Everyone who loves is born of God and knows God. 
4:12 If we love one another, then God abides in us, and His love has been perfected in us. 
4:13 By this we know that we abide in Him and He is us, because He has given us of His Spirit 
4:15 Whoever confesses that Jesus is the Son of God, then God abides in him, and He abides in God. 
5:1 Whoever believes that Jesus is the Christ is born of God, 
5:2 By this we know that we love the children of God,when we love God and keep his commandments. 
5:4 Whatever is born of God overcomes the world. 
5:18 Whoever is born of God do not sin.

Degree of Assurance

Unlike the doctrine of Eternal Security which is absolute, the Assurance of Salvation is relative to the degree to which one behaves as a child of God should. That is, while one can say Once Saved Always Saved, whether the "Once Saved" has occurred is in question - the certainty of which is relative to the evidence in one's life  that one has been saved. Evidences include both the exercise of one's faith and the impact regeneration (being born of God) has had on one's lifestyle, attitudes, character qualities.

Example:

1Thess:
3  We continually remember before our God and Father your work produced by faith, your labor prompted by love, and your endurance inspired by hope in our Lord Jesus Christ.
4  For we know, brothers loved by God, that he has chosen you,
5  because our gospel came to you not simply with words, but also with power, with the Holy Spirit and with deep conviction. You know how we lived among you for your sake.
6 You became imitators of us and of the Lord; in spite of severe suffering, you welcomed the message with the joy given by the Holy Spirit.
7  And so you became a model to all the believers in Macedonia and Achaia.
8  The Lord’s message rang out from you not only in Macedonia and Achaia— your faith in God has become known everywhere. Therefore we do not need to say anything about it,
9  for they themselves report what kind of reception you gave us. They tell how you turned to God from idols to serve the living and true God,
10  and to wait for his Son from heaven, whom he raised from the dead— Jesus, who rescues us from the coming wrath

While I say logically that Assurance is relative, this is not to say that one cannot be confident that they (or in this case the Thessalonians) are saved. Indeed verse 4 "we know" appears to express complete certainty. In the Greek "know" here is in the perfect tense (often English translations don't bring out such nuances). The Online Bible Greek Lexicon says of the perfect tense that is "describes an action which is viewed as having been completed in the past, once and for all, not needing to be repeated."

Consequently the kind of evidences mentioned above should invoke confidence, indeed certainty, of one's salvation status or in this case that of others.

Likewise there are behaviors which can invoke confidence, indeed certainty, that one is not saved.

Example:

1Cor 6:9,10 "Do you not know that the wicked will not inherit the kingdom of God? Do not be deceived: Neither the sexually immoral nor idolaters nor adulterers nor male prostitutes nor homosexual offenders nor thieves nor the greedy nor drunkards nor slanderers nor swindlers will inherit the kingdom of God."

And again "know" is in the perfect tense in the Greek.

An event in Acts 15 also mentioned in Gal 2, when Paul went to the church at Jerusalem to deal with an issue of men coming from that church and preaching a false gospel to the Galatians, he found men in leadership who promoted the heresy and said of them "This matter arose because some false brothers had infiltrated our ranks to spy on the freedom we have in Christ Jesus and to make us slaves."

Likewise throughout 1John we are taught how to identify false brethren.

But again the degree which one behaves as a child of God is the degree to which one can have assurance of the salvation status of oneself or others, and likewise to the degree which one does not behave has a child of God one can have assurance that such are not saved.

Jesus said, "If you were Abraham’s children, then you would do the things Abraham did. As it is, you are determined to kill me, a man who has told you the truth that I heard from God. Abraham did not do such things.  You are doing the things your own father does." "We are not illegitimate children," they protested. "The only Father we have is God himself." Jesus said to them, "If God were your Father, you would love me, for I came from God and now am here. I have not come on my own; but he sent me. Why is my language not clear to you? Because you are unable to hear what I say. You belong to your father, the devil, and you want to carry out your father’s desire. He was a murderer from the beginning, not holding to the truth, for there is no truth in him. When he lies, he speaks his native language, for he is a liar and the father of lies." John 8:39-44