‘And Peter answered, and said to Jesus, “Lord, it is good for us to be here. If it is your will I will make here three booths, one for you, and one for Moses, and one for Elijah.” '
Peter appears to have considered that this wonderful scene was something that was intended to be permanent, or at least strove to make it so, although Mark tells us that he also spoke out of fear, not knowing what to say. So we must not judge him too harshly. But what he says does demonstrate that to him at least what he was seeing was actually happening and not just a vision. For he suggested that he and his fellow disciples should build three booths, one for Jesus, one for Moses, and one for Elijah (for such great leaders could hardly be expected to build their own). His probable idea was that these booths would shield their glory from the people (see Luke 9:31) and act as sanctuaries to which people could come to consult with them. It may also have included the idea that as they had apparently come to assist Jesus in His work, they must therefore be given accommodation suited to their status (as tents of generals might be around that of the king). They would have been seen by Peter as useful men to have around. For both Moses and Elijah had been highly experienced in dealing with aggressors in their day, and Peter might have seen in their presence a hope of the fulfilment of his confession about the Messiah, without any suffering, which would lead to these mighty three acting to bring in the Kingly Rule of God. His view would be that such heavenly visitants could hardly fail to achieve their aims. And in his ignorance the last thing that he wanted was for them to leave. The mighty Peter who had been blessed by God with the revelation about Jesus' Messiahship, is now seen to be the foolish Peter whose ideas are ridiculous in the extreme. He is being taught that he has much to learn.
Possibly also there was the thought that the people would be able to come up the mountain and seek the wisdom of these three great teachers, and see in their presence the sign that up to this point Jesus had refused to give. Perhaps, Peter might have thought, this was what Jesus had been leading up to? His idea was probably that this would indeed then cause a stirring among the people and an establishing of the truth in their hearts, after which, led by these three ‘greats', the people would go forward to conquer the world. Their prayer of, ‘Your Kingly Rule come' would be dramatically answered (at this stage the Apostles were still looking for an earthly ‘kingdom' - Acts 1:6).
Compare how both James and John are thinking of Jesus in similar physical terms when they try to pre-empt Peter later for the positions at His right and left hand sides (Matthew 20:21), and how John will describe the two witnesses in Revelation 11:5-6 in terms which appear to have Moses and Elijah in mind, although by then his ideas had been straightened out and he recognises their secondary position and that Jesus' throne and kingship is in Heaven, so that their presence simply leads up to the Rapture and the final judgment, pictured in vivid terms.
There was, of course, in this idea of Peter's a diminishing of the status of Jesus which Peter apparently did not appreciate, but he was soon to be made aware of it in the voice that followed, which would single out Jesus as unique, and greater than Moses and Elijah, as the One Who alone was to be listened to. Moses and Elijah were of the past. The future lay with Jesus and His words. He would not share His glory with another. (Nor indeed could they share it).