The Ministry of Jesus leading up to the Transfiguration, was generally that of a teacher and healer, going about Galilee doing good.
After the Transfiguration the ministry of Jesus focuses on to getting Jerusalem and fulfilling his purpose and mission – the Cross and Resurrection.
The three gospels all have the Transfiguration of Jesus as the turning point in the mission and ministry of Jesus. There is a sense in which in this moment with Peter and James and John, there is a revelation which on the surface looks to suggest that the purpose of the event is to affirm and validate not simply the Divine Commission, but in fact the divine nature of Jesus himself. The point of the appearance with Moses and Elijah is that Jewish exegesis required two witnesses, from the Law and the Prophets, which is what Moses and Elijah represent.
At the same time there is, as we all know when we are facing something difficult a need for there a vision of what we are embarking upon, a goal, so that we can keep focused in the meantime. What keeps the student going through the long years of study is the goal of accomplishment.
It seems this moment, this glimpse of glory, at once affirmed his divinity, yet in a real sense affirmed his humanity, in that like us he needed to see the goal to fix his focus and get on through the tough times with his vision clearly caught on the goal.
For the ministry of Jesus the Transfiguration appears is a game changer, a circuit breaker, very much as for us as Christians the encounter with Jesus is a game changer and a circuit breaker.