“In all things, love.” –I Corinthians 16:14
That’s one test of a believer and a mighty important one it is. Our Lord said it is the mark of a disciple. (John 13:34-35)
Look for the love. Otherwise, you know this one with whom you are discussing scriptures and doctrines is no follower of Jesus.
The cultist you’re talking religion to across the table or across the continent feels no need to love you since he/she has decided you are not a follower of Jesus since you disagree with their doctrine. I’ve sat at a table with a Jehovah’s Witness who was brutal and mean-spirited and who may as well have thought of me as a child-molester by the scoffing and belittling he was dishing out. (I was a younger pastor, and had not learned that there comes a time when it’s all right to say, “This meeting is over,” and walk out.)
But while love is the first mark of the believer, there’s another test for determining whether the person across the table is an honest seeker.