Why did Jesus allow Peter, James and John to experience the moment of his Transfiguration? It showed them the passion leads to the resurrection. In a few weeks, we’ll see Jesus go to Jerusalem, suffer and die on the cross.
Jesus wanted to bolster the faith of Peter, James and John so that when they were to see him die on the cross, they would remember the resurrection, the glory that was to follow.
Only John was at the foot of the cross, perhaps he was the only one who remembered.
Never doubt in the darkness what God showed you in the light.