Gregory the Great — as excerpted in the Catena Aurea on John 8:57-59

“(ut sup.) Our Saviour mildly draws them away from their carnal view, to the contemplation of His Divinity; Jesus said unto them, Verily, verily, I say unto you, Before Abraham was, I am. Before is a particle of past time, am, of present. Divinity has no past or future, but always the present; and therefore He does not say, Before Abraham was, I was: but, Before Abraham was, I am: (Exod. 3:14) as it is in Exodus, I am that I am. Before and after might be said of Abraham with reference to different periods of his life; to be, in the present, is said of the truth only.”