The Joy of Jesus is Transformational

Christ was transfigured before them.
The word “transfigured" means to change, transfigure, transform.

Rom 12:2 And be not conformed to this world: but be ye transformed by the renewing of your mind, that ye may prove what is that good, and acceptable, and perfect, will of God.

2 Cor 3:18 But we all, with open face beholding as in a glass the glory of the Lord, are changed into the same image from glory to glory, even as by the Spirit of the Lord.

Christians are changed-renewed from within from glory to glory. Our transformation is a “gradual” experience, and will be completed when Christ appears at the Rapture.

Christians are being inwardly changed day by day. At the Rapture, Christians will be “outwardly changed”—as was Christ on the mount—in the twinkling of an eye.

1 Cor 15:52 In a moment, in the twinkling of an eye, at the last trump: for the trumpet shall sound, and the dead shall be raised incorruptible, and we shall be changed.

I Jn 3:2 Beloved, now are we the sons of God, and it doth not yet appear what we shall be: but we know that, when he shall appear, we shall be like him; for we shall see him as he is.

Elijah
Some confusion here. The three apostles believe Jesus is the Christ (Messiah) but why then do the scribers (keepers of the law) say that Elijah must come first? John the Baptist himself said, “I am not Elijah”. (the resurrected Old Testament prophet) This was enough for the unspiritual Jews who were merely looking for signs to reject Jesus as the Messiah, but to the spiritually discerned, John had already come. John the Baptist came in the Spirit and power of Elijah.

The angel speaking to Zacharias, the father of John the Baptist.
Luke 1:17 And he shall go before him in the spirit and power of Elias, to turn the hearts of the fathers to the children, and the disobedient to the wisdom of the just; to make ready a people prepared for the Lord.