Jehovah Witnesses and Mormons do not believe that Jesus--the man--was God.  So when they knock on my door, I greet them with my Bible and say, “I know you mean well, but the Apostles  saw him after he rose from the dead, and they testified that Jesus was God.”

And then I open my Bible to 2nd John 1:10 and read, “If anyone come unto you and brings not this doctrine, (that Jesus is God)  do not receive him into your house, neither bid him God speed: for he that bids him God speed is partaker of his evil deeds.”

Jesus is God.  He lived, he died, he rose from the dead to intercede for us with the Father.  He paid for our sin.  That is the doctrine--anything else is  considered to be false.  John goes so far as to say that spreading a false doctrine is an evil deed.

They may be good people.  Well-intentioned.  But they do not spread the true gospel of Christ.  And it harms those who accept a false idea about Jesus.  Only God can conquer death.

I don’t argue, the scripture gives me the authority to speak up.  I close the door, and hope they re-read their Bibles.  We have become so afraid of offending people that we sometimes don’t stand up for truth.  You don’t have to argue with them, just read the Scripture.