Why did Jesus show Himself transfigured?

He led them up the mountain and showed them
His kingship before His passion, and His power before His death,
His glory before His disgrace, and His honor before His dishonor, so that,
when He was arrested and crucified by the Jews,
they might know that He was not crucified through weakness,
but willingly by His good pleasure
for the salvation of the world.

St. Ephrem the Syrian