I’m thankful you’re concerned for your friend, and I hope you will pray for him. He may or may not be seeking God, but God is seeking him, and God wants to use you to point him to Jesus.
My first suggestion is a negative one: Try to avoid getting involved in debates over the good or bad points of various religions. Elsewhere in your letter, you indicate that your friend has made a study of the world’s major religions, and he probably knows much more about them than you do. Therefore, arguing about them probably won’t be helpful.
Instead, urge him to study the life of Jesus Christ as it is found in the Gospels of the New Testament. The reason is because Jesus made a startling claim that sets him apart from every other religious leader who ever lived: He claimed that he was God in human flesh. He declared, “I and the Father are one” (John 10:30).
Is his claim true? Yes, and he proved it by rising from the dead. You can visit the tombs of other religious leaders — but not that of Christ. He rose from the dead by the power of God, proving for all time that heaven’s door has been opened for us.
Yes, challenge your friend to look at Christ. But may his questions challenge you to live more completely for Christ, and to be an example to others of his compassion and truth.