Jamieson, Fausset and Brown on Exodus 34:29

29. Moses wist not that the skin of his face shone while he talked with him--It was an intimation of the exalted presence into which he had been admitted and of the glory he had witnessed (2Co 3:18); and in that view, it was a badge of his high office as the ambassador of God. No testimonial needed to be produced. He bore his credentials on his very face; and whether this extraordinary effulgence was a permanent or merely temporary distinction, it cannot be doubted that this reflected glory was given him as an honor before all the people.