The notion that anyone stops being a sinner is false to begin with.
Paul shows indwelling sin as a present reality in his flesh, twice in Romans 7:17-21, concluding that evil was present with him.
John shows us that we can not say we have, present tense, no sin, or the truth is not in us. 1 John 1:8
Paul also shows us that we are no better than the unsaved as believers, Romans 3:9
Paul claimed the opposite, that he was the chief of sinners AFTER salvation, using present tense "I am." 1 Tim. 1:15
It is from understanding this state we come to understand the critical need of God's Mercy in Christ, His forgiveness, and Grace.
and we are well advised to not be the SLAVE OF SIN. Enslavement starts by not recognizing our state, and being TRUTHFUL about but instead, we are turned to liars and hypocrites by SIN when we deny our own condition in and of the flesh, which is factually in opposition and contention to THE SPIRIT. Gal. 5:17