The gateway to the Kingdom of God is the Cross...

[...] The Transfiguration of Christ is the preparation for the Cross. And the Cross of Christ is the starting-point of His glory as a person. With His Transfiguration, Christ didn’t acquire anything new, but gave his disciples strength before His Crucifixion. The disciples needed to be strengthened so that they could come to terms with the Crucifixion of their Teacher, and with their own cross which they bore later in the name of their Teacher.