Jamieson, Fausset and Brown on Exodus 34:5

5. the Lord descended in the cloud--After graciously hovering over the tabernacle, it seems to have resumed its usual position on the summit of the mount. It was the shadow of God manifest to the outward senses; and, at the same time, of God manifest in the flesh. The emblem of a cloud seems to have been chosen to signify that, although He was pleased to make known much about himself, there was more veiled from mortal view. It was to check presumption and engender awe and give a humble sense of human attainments in divine knowledge, as now man sees, but darkly.