Genesis 15:12

A deep sleep - תרדמה tardemah, the same word which is used to express the sleep into which Adam was cast, previous to the formation of Eve; Genesis 2:21.

A horror of great darkness - Which God designed to be expressive of the affliction and misery into which his posterity should be brought during the four hundred years of their bondage in Egypt; as the next verse particularly states.

And the sun was about to set. - This visit of the Lord to Abram continues for two nights, with the intervening day. In the former night he led him forth to view the stars Genesis 15:5. The second night sets in with the consummation of the covenant Genesis 15:17. The revelation comes to Abram in a trance of deep sleep. The Lord releases the mind from attention to the communications of sense in order to engage it with higher things. And he who makes the loftier revelation can enable the recipient to distinguish the voice of heaven from the play of fancy.

15:12 And when the sun was going down — About the time of the evening oblation. Early in the morning, while the stars were yet to be seen, God had given him orders concerning the sacrifices, Genesis 15:5, and we may suppose it was at least his morning's work to prepare them, and set them in order; which when he had done, he abode by them praying and waiting 'till towards evening.

A deep sleep fell upon Abram — Not a common sleep through weariness or carelessness, but a divine extasy, that being wholly taken off from things sensible, he might be wholly taken up with the contemplation of things spiritual. The doors of the body were locked up, that the soul might be private and retired, and might act the more freely.

And lo, a horror of great darkness fell upon him — This was designed to strike an awe upon the spirit of Abram, and to possess him with a holy reverence. Holy fear prepares the soul for holy joy; God humbles first, and then lifts up.