2Jn 1:9-11 BSB 9 Anyone who runs ahead without remaining in the teaching of Christ does not have God. Whoever remains in His teaching has both the Father and the Son. 10 If anyone comes to you but does not bring this teaching, do not receive him into your home or even greet him. 11 Whoever greets such a person shares in his evil deeds.

This could be a Mormon missionary, Jehovah's Witness, or even the local Baptist preacher. We know they don't know the truth, so we are not even to acknowledge them.