When a person accepts Jesus into their heart and accepts what Jesus did on the cross for them they are sealed (end of story)
2Corinthians 1:22 who also has sealed us and given us the Spirit in our hearts as a guarantee.
Ephesians 1:13 In Him you also trusted, after you heard the word of truth, the gospel of your salvation; in whom also, having believed, you were sealed with the Holy Spirit of promise,
Ephesians 4:30 And do not grieve the Holy Spirit of God, by whom you were sealed for the day of redemption.
Yet Christians still do things that are sinful as even the apostle Paul so clearly pointed out....
1 Corinthians 8:6 yet for us there is one God, the Father, of whom are all things, and we for Him; and one Lord Jesus Christ, through whom are all things, and through whom we live. 7 However, there is not in everyone that knowledge; for some, with consciousness of the idol, until now eat it as a thing offered to an idol; and their conscience, being weak, is defiled. 8 But food does not commend us to God; for neither if we eat are we the better, nor if we do not eat are we the worse. 9 But beware lest somehow this liberty of yours become a stumbling block to those who are weak. 10 For if anyone sees you who have knowledge eating in an idol’s temple, will not the conscience of him who is weak be emboldened to eat those things offered to idols? 11 And because of your knowledge shall the weak brother perish, for whom Christ died? 12 But when you thus sin against the brethren, and wound their weak conscience, you sin against Christ.
Ephesians 4:25 Therefore, putting away lying, “Let each one of you speak truth with his neighbor,” for we are members of one another. 26 “Be angry, and do not sin”: do not let the sun go down on your wrath, 27 nor give place to the devil.
Romans 4:7 “Blessed are those whose lawless deeds are forgiven, And whose sins are covered; Blessed is the man to whom the LORD shall not impute sin.”
1Corinthians 6:17 But he who is joined to the Lord is one spirit with Him. 18 Flee sexual immorality. Every sin that a man does is outside the body, but he who commits sexual immorality sins against his own body.
Titus 3:8 Reject a divisive man (in reference to a brother in Christ) after the first and second admonition, 11 knowing that such a person is warped and sinning
Yet because we are sealed with the Holy Spirit we can recognize ourselves dead to sin and alive to God.
Romans 6:11 Likewise you also, reckon yourselves to be dead indeed to sin, but alive to God in Christ Jesus our Lord.
Romans 6:13 And do not present your members as instruments of unrighteousness to sin, but present yourselves to God as being alive from the dead, and your members as instruments of righteousness to God.