When you lose something and find it back, you rejoice more than if you never lost it.
So with sinners who leave the Lord, but then return.
Those who never left should rejoice with God instead of condemn.
Once a man had a son who took his inheritance and wasted it all on immoral living. He landed in the gutter and figured he had nowhere to go but up with his father. He returned and repented to him, and the father welcomed him back with celebration. The older brother resented father's welcome, but the father pleaded with him to extend grace to his brother.
The Gospel of Jesus is all about restoring what was lost to its rightful owner (sinner to God) and rejoicing over it. Do both! Restore (yourself and others) and rejoice!