Thank you for this question. Somehow, I failed to bring the reader's attention to this important episode when we spoke of the biblical roots of the veneration of saints. Here is the text:
[1] And after six days Jesus taketh unto him Peter and James, and John his brother, and bringeth them up into a high mountain apart: [2] And he was transfigured before them. And his face did shine as the sun: and his garments became white as snow. [3] And behold there appeared to them Moses and Elias talking with him. [4] And Peter answering, said to Jesus: Lord, it is good for us to be here: if thou wilt, let us make here three tabernacles, one for thee, and one for Moses, and one for Elias. [5] And as he was yet speaking, behold a bright cloud overshadowed them. And lo, a voice out of the cloud, saying: This is my beloved Son, in whom I am well pleased: hear ye him. [6] And the disciples hearing, fell upon their face, and were very much afraid. [7] And Jesus came and touched them: and said to them, Arise, and fear not. [8] And they lifting up their eyes saw no one but only Jesus. [9] And as they came down from the mountain, Jesus charged them, saying: Tell the vision to no man, till the Son of man be risen from the dead. (Matthew 17, similar Mark 9:1-8).
What do we see here? First, it is a select group of Apostles, and they go up the mountain; many times Jesus goes up the mountain to fast and pray alone, but this time he takes three select apostles along. So, we take it, it is a difficult lesson requiring an elevated mind. Indeed, after the lesson is done, the three apostles are told not to tell anyone of it till the Resurrection.
So what is the lesson? Jesus is seen transfigured, indicating His future glorified body. Elijah and Moses are not transfigured. This then shows their presence in heaven even before the General Judgment that we all await: it is Jesus and the two Hebrew saints in their condition now.
Jesus is conversing with the saints. This, I underscore, means the communication was two-way. That is exactly how we understand the Communion of saints to be: a single family where a saint from time to time takes the initiative to speak. The nature of the communication is not narrated: apparently the purpose of the lesson is not what Jesus and Moses and Elijah were talking about, but that the conversation is possible. This is how we understand the communion of saints to be also: adoration is made of Christ, petitions are given to Christ, instructions are received from Christ, -- the communion of saints has every channel of communication open between the saints and Christ.
The Holy Apostles do not engage in the conversation. Otherwise, it would have been a direct proof-text of veneration of Hebrew saints. The fact that they are told to keep the experience to themselves till the Resurrection underscores to us that the veneration of saints was not desirable prior to the Church Age, even to the extent that it was possible. Indeed we see that they were struck with fear: that would not be an emotion conducive to venerating anyone. Indeed, Leviticus 20:27 directly forbids any kind of behavior that they see Jesus engaged in. Of course they would fear: they see their Lord breaking the Jewish law and asking them to come along.
But they also are in the presence of grace. "It is good to be here"-- St. Peter exclaims. And indeed it is good to venerate a saint: it give your soul a comfort of knowing your future heavenly company; it restored the believer's hope in his own salvation and invaluable advice, one that cannot be gotten from a person whose sanctification is yet incomplete, comes from a conversation with a saint. It is so good that Peter wants to build three tabernacles for Jesus, Moses and Elijah.
A tabernacle is a portable shrine, but to a Jew it is also the home of the Ark of the Covenant. Here blessed Peter with his boyish nature is about to get himself in trouble: he apparently thinks that either Jesus is a mere prophet like the other two, or that all three are Gods. Promptly his mistake is corrected: it is Jesus alone that is to dwell in the tabernacle. Here, by the way, how the Tabernacle of Jesus looks:
The theophany occurs next: God appears on the cloud to answer the worshipful impulse of Peter. God makes it clear that Christ alone we worship and indeed the conversation is no longer; the saints withdraw. The undeveloped, still profoundly Jewish religious development of Peter made the conversation, to which he was invited to, no longer productive or possible.
Christ tells the Apostles to keep the event secret till the Resurrection. Implied in that is the commandment to tell everyone after the Resurrection. It was the desire of Our Lord to gradually teach His people to properly venerate saints, to properly separate veneration from worship and to experience divine grace without fear. That, indeed, He also told the Holy Apostles: "Arise, and fear not".
This is a Holy Icon of the Transfiguration in Byzantine tradition:
Is there another lesson? The Transfiguration is also a feast of the threshold of our blessed Church Age: Christ rises from among the Jewish prophets and is with God; soon the Jewish religion will be replaced by the new faith where we converse with saints and with God without fear. It is also a lesson of discipleship: the light of the Mount Tabor that struck even the Holy Apostles may become the gentle light inside, borne by the Catholic believer, the light that shines and illuminates our lives and drives the heresies away.