Both of these have serious problems:
Saints who think they are no longer sinners.
Sinners who think they cannot become saints.
The problem with saints who think they are no longer sinners is that they do not take sin seriously. In most cases, it is their own sin that they do not take seriously. They may in fact take the sins of others very seriously. They may be experts at pointing it out and pontificating on the seriousness of their (other peoples’) sins. But they either will not admit, or completely overlook their own sin. And that is a problem. Because we are all sinners. And one of the worst kind of sinner is the religious type who will not admit to or address their faults. They deserve the label “hypocrite,” and they do damage to the mission and ministry of the church.
The problem with sinners who think they could never be saints is that they do not understand grace sufficiently. They do not believe that God would ever willingly and lovingly involve Himself in a life like theirs. They may think they are beyond the reach of forgiveness, that they are unredeemable. They know that they don’t have the moral resources or will-power to raise themselves to the level of sainthood. But they also have not grasped how grace is purest when it reaches the lowest, and that it is truest when it helps the truly helpless, and that it is noblest when we cannot return the favor, but only receive God’s kindness as a gift.
We are at our best when we, as sinners, accept the gift of being saints. And we are at our best when we as saints do not forget that we are still sinners.