A believer who has sex outside of marriage as one example.
People fail to realize that we judge things everyday of our lives. So many things are judgment calls.
And you are right. God gave things like the Ten Commandments, but how are they to be applied if a person cannot “judge” another? There’s no way of confronting sin without having the ability to compare it with God’s righteous standard and declaring it to not meet that, and that is, by default, judging.