The Burning Bush

And there appeared unto him the angel of the Lord in a flame of fire out of a bush; and he saw that the bush burned with fire, but the bush was not consumed. And Moses said, I will pass over and see this great sight, why the bush is not consumed.

Exodus 3:2-3, as St Cyprian of Carthage gives it

He came in likeness of fire on the bush in the wilderness, and the fire kept playing on the shrub yet was it not consumed. And Moses marvelled at the sight. Yet how is not a tree a thing that has no alliance with fire? and how is the readily consumed wood patient of the onslaught of flame? But this matter was (as I said) a type of a mystery, which exhibited endurable to the measures of the human nature, the Divine Nature of the Word, at His Will, for to Him is nothing impossible.

Cyril of Alexandria, That Christ is One