I was just wondering whether or not there are some good/reliable extra-biblical sources for the divine nature of Christ, which I could pose to a few atheist friends.
There are many but I will give you two. In John 8:56-58 Jesus said, "Your father Abraham rejoiced to see My day (when Jesus was to be born on this earth), and he saw it and was glad. Then the Jews said to Him, “You are not yet 50 years old, and have You seen Abraham?” Jesus said to them, "Most assuredly, I say to you, before Abraham was, I AM (which was the name God had given to Moses on the OT) and all of the transfigured Gospel stories but Matthew has a better record in Matthew 2:2-3, “and He was transfigured before them. His face shone like the sun, and His clothes became as white as the light. and behold, Moses and Elijah appeared to them, talking with Jesus.” Jesus often hides His Divinity so that by faith you can discover it yourself. When you discover this Jesus to be both Divine and Human than you have the privilege to talk to Him as did Moses and Elijah. If I can give another account it is in John’s Gospel with Jesus meeting with the Samaritan woman at a well. When Jesus asked her to tell her husband she responded that she had no husband and He said that she spoke the truth because He told her that she had 5 husbands and the man living with her now is not her husband. Now how did Jesus know about this? The woman changed to discover this man Jesus is much more.