"At that time they will see the Son of Man coming in a cloud with power and great glory." (Luke 21:27)
This text in Luke is a direct reference to the prophecy in Daniel:
"I beheld therefore in the vision of the night, and lo, one like the son of man came with the clouds of heaven, and he came even to the Ancient of Days: and they presented him before him." (Daniel 7:13)
Clouds in the Hebrew Scriptures indicate the appearance of the Lord. His presence in the tabernacle and in the temple is signified by the presence of a cloud (Exodus 40.34-35; 1 Kings 8,10-11; 2 Chronicles 5,13-14). The pillar of cloud also indicates the Lord's presence in Exodus (13:21-22, 14:19).ln Deuteronomy (5:22), the Lord's presence on Sinai is connected with fire, cloud, gloom and darkness. There are further references in Jeremiah, Ezekiel and the Psalms. The coming of the Lord in judgment on the Day of the Lord is with clouds, a figure picked up in the Bookof Revelations.
No being, other than the Lord God, appears with clouds in the Scriptures, not even angels.
Thus, the presence of 'a cloud' is a direct referene to the presence of God, a Divine image, not an angelic or prophetic one.
Note also that at the Transfiguration, Christ is seen flanked by Elias and Moses (signifying the Law and the Prophets) in a dazzling cloud atop the mountain.
Clouds, as an image, serve a dual purpose.
The one is that they obscure, they are a type of veil, insubstantial, and yet impenetrable. In Luke, in the Greek, the Son of Man comes 'with' the clouds of heaven, but we cannot be sure (from the original Aramaic via the Greek, the conjunction is variable) whether He is 'with', or 'in', or 'on' ...
The other is that the cloud denotes not so much something that hides, but rather something without physical form, the Formless, and as such the mental state of knowing, the eye of the mind, is occluded, not because the cloud masks God, but rather there is nothing but the Divine Darkness of a Superabundance of Light — in this sense clouds signify that which is Beyond Knowing.
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A reading of the Son of Man and the Ancient of Days could, from a retroactive, Catholic perspective, point to this text in Daniel as a prophecy of the Incarnation, of the Second Person of the Holy Trinity, but that would be to read too much into the Prophet.