If you account him a believer, though greatly mistaken in the subject of debate between you, the words of David to Joab, concerning Absalom, are very applicable: "Deal gently with him for my sake." The Lord loves him and bears with him; therefore you must not despise him, or treat him harshly. The Lord bears with you likewise, and expects that you should shew tenderness to others, from a sense of the much forgiveness you need yourself.
In a little while you will meet in heaven; he will then be dearer to you than the nearest friend you have upon earth is to you now. Anticipate that period in your thoughts; and though you may find it necessary to oppose his errors, view him personally as a kindred soul, with whom you are to be happy in Christ for ever.http://www.opc.org/nh.html?article_id=217