Judge not, that ye be not judged
Think before you criticize others, for you may be criticized yourself in return.
The source is Matthew, vii, 1. It is part of the sermon of the mount and reads:: ‘Judge not, that you may not be judged yourselves; for as you judge so you will be judged, and the measures you deal out to others will be dealt out to yourselves.’
Matthew 18:15-17
New International Version (NIV)
Dealing With Sin in the Church
15 “If your brother or sister[a] sins,[b] go and point out their fault, just between the two of you. If they listen to you, you have won them over. 16 But if they will not listen, take one or two others along, so that ‘every matter may be established by the testimony of two or three witnesses.’[c] 17 If they still refuse to listen, tell it to the church; and if they refuse to listen even to the church, treat them as you would a pagan or a tax collector.
The bible commands me to judge. I believe the Catholic interpretation of not judging refers to not judging if someone will go to Hell. We can judge the outward appearances (e.g. outwards displays of objective sin) in an effort to turn the person away from sin but we can’t say someone will be damned to Hell.