I was reading about the transfiguration in Matthew and I realized that Peter seems to know immediately that the two shining figures with Jesus are Elijah and Moses. How did Peter know what Elijah and Moses looked like since no one in the first century would know what they looked like and there were no photos of them?
According to Matthew’s account, Moses and Elijah were “talking with him.” Him, here is Jesus. What we do not know is the content of their conversation. Perhaps Jesus mentioned Moses’ name or Elijah’s name. Either that, or the details of their conversations made it clear who these two were. Perhaps Jesus said, “Moses, it is great having you here.” We cannot know the content of the conversation, but, from what Peter said, we can assume that the content was such that the apostles knew who Jesus was speaking with. You said that they seemed to know immediately that the two were Elijah and Moses, but this is not what is implied by the actual text, which has a conversation occurring before Peter speaks up.