In every culture, there are important symbols that carry significant value. Some of the important elements are places or substances. For the Israelite community, one such place is a mountain. The moment you hear of a mountain; you should immediately suspect something momentous. Momentous in this sense, refers to an event that carries great importance or significance, especially in its bearing of the future. In every case, Jewish legendary events unfold on the mountain.
In case of Moses, it was Mt. Sinai, where the Theophany unfolded. Theophany is the literal translation of the Greek – “the appearance of a god or of God.” (Theos – god & Phainein – to show). He went up, six days, and on the 7th day, again, a traditional symbol of perfection, he was called out of the cloud. Matthew does the same. These are typical Jewish traditionally significant narratives. In Matthew, key events have to happen on the mountain, no matter what.
In this case, the Transfiguration, which is the climax of the epiphany, revealing Jesus’ identity. Jesus is revealed to the disciples and it had to be a momentous event. It is huge, if not stupendous (something that causes astonishment or wonder).