But now apart from the Law the righteousness of God
has been manifested, being witnessed by the Law and
the Prophets, even the righteousness of God through
faith in Jesus Christ for all those who believe...
For as through the one man's disobedience the many
were made sinners, even so through the obedience
of the One the many will be made righteous.
Romans 3:21-22a, 5:19
Saints is the dominant term in the NT for those who have placed their faith in the person of Christ, and in His finished work on their behalf. So why are those who come to faith in Christ called saints (hagiois, holy ones)? Because they have been made righteous in Christ, they have received the righteousness that comes from God (God's righteousness, Christ's righteousness) as a result of their faith. This is why the Scriptures never call those who have come to faith in Christ sinners. Yes, while we were sinners (those who practice sin, those who live to sin, those who have a lifestyle of sin, those who spurn obedience to God, those who are called the ungodly) Christ died for us. Christ did die for the ungodly (a synonym for sinners), but post-salvation we are sinners no longer, we are now saints. Saints are those who are holy (I Corinthians 3:17), those who practice righteousness (I John 3:7-11), and those who have been freed from sin (Romans 6:6-7, 22).
Yes, we are saints, God's holy ones; and yes we still sin; but we no longer live in sin, and sin no longer dominates our lifestyle. Instead of practicing sin, we have incidents of sin. God has cleansed us of our sin (John 13:5-11, 15:3, I Corinthians 6:9-11, Ephesians 5:26, Titus 3:5) so that we stand holy and clean before Him, and each time we confess our sins He cleanses us thoroughly (John 13:10, I John 1:9) Our standing before God, our position before God, is one of holiness, the holiness of Christ. That is why He calls us His holy ones, and has called us to be saints (Romans 1:7, I Corinthians 1:2, II Timothy 1:9, Hebrews 3:1).
So now fellow saints, let us be like the Holy One who called us, let us be like His Son (the Holy One of God), let us be holy in all our behavior, for this is what we are and who we are called to be.