All repentant people were once sinners, but they turned from sin and became non-sinners...thanks to the grace and mercy of God.
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"Therefore if any man be in Christ, he is a new creature: old things are passed away; behold, all things are become new." (2 Cor 5:17)
According to your vicious heresy, a new creature has been a sinner. When God created Adam, He created someone who had never been a sinner. Yet, according to your heresy, when God creates the new creature spoken of by Paul in the verse you quoted, He created someone who
has been a sinner.
By your heresy, you reject and spit upon the Pauline doctrine of God's new creatures in Christ, by telling us that God's new creatures in Christ
"were once sinners". You are saying God's work in creating His new creations in Christ is not as good as was His work in creating Adam; for, a creature that was once a sinner is not as good as a creature that was never a sinner.