If any group of Christians deserved to be called “sinners”, it ought to have been the Corinthian Christians. However, the Apostle Paul never—that’s right, never—uses the label “sinner” to identify any of them. How then does Paul identify and address them (emphasis mine)?
To the church of God that is in Corinth, to those sanctified in Christ Jesus, called to be saints together with all those who in every place call upon the name of our Lord Jesus Christ, both their Lord and ours: (1 Corinthians 1:2, ESV)
To the church of God that is at Corinth, with all the saints who are in the whole of Achaia: (2 Corinthians 1:1b, ESV)
This is in quite a contrast to many contemporary preaching and teaching that consistently tries to reinforce the idea that Christians are “sinners saved by grace” and similar phrases. In all cases, the implication is that the core identity of Christian is still “sinner” whose rottenness is just covered over by “Christ’s righteousness”. If that covering disappears or is taken off, what is revealed is what was there all along.
I believe anytime a preacher or teacher uses “sinner” to identify Christians, be it themselves or their audience, it is a false gospel message. It demeans the work of Christ. Not only that, it conveys the message that the power of the cross of Christ is/was not enough.
I used to accept this idea as true, that it was a statement of humility, that this kind of talk elevated the grace and power of God to save. Now I see it as a message, if not from the devil, then something very close to it. A message that cannot change the core identity of persons has no power at all. It is a false gospel.
Both Jesus and Paul are quite clear that “sinner” refers to those who have not yet experienced the redeeming and restoring power of God. Those who have, have had their core identities changed. They are no longer “sinner” but “saint”. This is the foundation of Paul’s appeals to the Corinthian believers: because they are no longer sinners, they really do have the power to behave in Christ-like ways. If Paul thought they were still sinners, he could not make that appeal—he would have written very different letters. For both Jesus and Paul, a “sinner” is a slave to sin and has no power to do anything other than live out their condition. A “saint” is no longer a “sinner” and therefore has the power to choose to live the way of Christ.
None of this is to say that Christians, therefore, do not or cannot behave in ways that are sinful. Far from it. Christians are saints who continue their struggle with their former habits resulting from sin. Saints are even free to choose to revert to their former ways (which is what Paul sees many of the Corinthians having done, and thus writing them a reminder and a warning, but even that choice does not change their identity!).
Some readers here may object on the grounds of a few passages that appear to identify Christians as “sinners”. These are primarily found in non-Pauline epistles (Hebrews, James), and one sentence found in 1 Timothy 1:15. A more comprehensive discussion of this overall topic that includes some of these texts can be found here: “Sinners” Who Are Forgiven or “Saints” Who Sin?—Robert Saucy
 I try to parse vocabulary here in a nuanced fashion. I define sin as being in a state of anti-God. Behaviors, actions, thoughts, etc. are not in themselves sin, but rather effects of the power of sin (c.f., Romans 1:18-2:11), i.e., sinfulness.