Atheists love to point out when Christians label a visible member of their tribe who’s screwed up by saying, “Well, if s/he did that, s/he was no Christian.” That is pathetic.
On the other hand, it’s often true. Don’t take it from me -
Jesus said, “Not everyone who says to me, “Jesus is my Lord,” will enter the kingdom of heaven, but only he who does the will of my Father who is in heaven. Many will say to Me on that day,
“Lord did we not prophesy in your name, and in your name drive out demons and perform many miracles?”
Then I will tell them plainly, “I never knew you. Get away from Me, you evil doer.”