The New Testament letters in particular have a great deal to say about Christian holiness. And as I such we cannot comprehensively study all the applications of holiness here, but I will touch on a few.
We who belong to Christ are in a battle against the sinful passions and desires inherent in our flesh. "Those who belong to Christ Jesus have crucified the sinful nature (the flesh) with its passions and desires." Gal 5:24 And "You were taught, with regard to your former way of life, to put off your old self, which is being corrupted by its deceitful desires" Ephesians 4:22 "Put to death, therefore, whatever belongs to your earthly nature: sexual immorality, impurity, lust, evil desires and greed, which is idolatry." Col 3:5 Sanctification is not contrary to God's grace, "for the grace of God that brings salvation has appeared to all men. It teaches us to say "No" to ungodliness and worldly passions, and to live self-controlled, upright and godly lives in this present age while we wait for the blessed hope— the glorious appearing of our great God and Savior, Jesus Christ, who gave himself for us to redeem us from all wickedness and to purify for himself a people that are his very own, eager to do what is good." Titus 2:11-14
To mention just a few areas of developing holiness:
"Get rid of all bitterness, rage and anger, brawling and slander, along with every form of malice." Eph 4:31 "But now you must rid yourselves of all such things as these: anger, rage, malice, slander, and filthy language from your lips." Col 3:8
"Do not let any unwholesome talk come out of your mouths, but only what is helpful for building others up according to their needs, that it may benefit those who listen." Eph 4:29 "Therefore each of you must put off falsehood and speak truthfully to his neighbor, for we are all members of one body." Eph 4:25 "Do not lie to each other, since you have taken off your old self with its practices." Col 3:9 "He who works deceit shall not dwell within my house; He who tells lies shall not continue in my presence." Ps 101:7
Some steal just because they're too lazy to work. "He who has been stealing must steal no longer, but must work, doing something useful with his own hands, that he may have something to share with those in need." Eph 4:28 More generally people steal because they covet other people's property. "You shall not covet your neighbor’s house; you shall not covet your neighbor’s wife ... nor anything that is your neighbor’s." Ex 20:17
"Do not be idolaters, as some of them were; as it is written: 'The people sat down to eat and drink and got up to indulge in pagan revelry.'" 1Cor 10:7
"Finally then, brethren, we urge and exhort in the Lord Jesus that you should abound more and more, just as you received from us how you ought to walk and to please God; for you know what commandments we gave you through the Lord Jesus. For this is the will of God, your sanctification: that you should abstain from sexual immorality" 1Thess 4:1-3
"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." 1Cor 6:9,10
"The acts of the sinful nature are obvious: sexual immorality, impurity and debauchery; idolatry and witchcraft; hatred, discord, jealousy, fits of rage, selfish ambition, dissensions, factions and envy; drunkenness, orgies, and the like. I warn you, as I did before, that those who live like this will not inherit the kingdom of God." Gal 5:19-21
These are the kinds of things that are important to God and therefore important to developing a healthy relationship with God.