John Calvin Commentary Genesis 15:10

"And he took him all these, and divided them in the midst, and laid each half over against the other: but the birds divided he not." — Genesis 15:10 (ASV)

And divided them in the midst. So that no part of this sacrifice may be without mystery, certain interpreters weary themselves in fabricating subtleties; but it is our task, as I have often declared, to cultivate sobriety. I confess I do not know why he was commanded to take three kinds of animals besides birds, unless it was that by this variety itself, it was declared that all the descendants of Abram, of whatever rank they might be, were to be offered up in sacrifice, so that the whole people, and each individual, would constitute one sacrifice.

There are also some things about which, if anyone curiously seeks the reason, I will not be ashamed to acknowledge my ignorance, because I do not choose to wander in uncertain speculations. Moreover, this, in my opinion, is the essence of the matter: that God, in commanding the animals to be killed, shows what will be the future condition of the Church.

Abram certainly wished to be assured of the promised inheritance of the land. Now he is taught that it would begin from death; that is, that he and his children must die before they would enjoy dominion over the land. In commanding the slaughtered animals to be cut into parts, it is probable that he followed the ancient rite in forming covenants, whether they were entering into any alliance or mustering an army—a practice that also passed over to the Gentiles.

Now, the allies or the soldiers passed between the severed parts so that, being enclosed together within the sacrifice, they might be more sacredly united in one body. That this method was practiced by the Jews, Jeremiah bears witness (Jeremiah 34:18), where he introduces God as saying, They have violated my covenant, when they cut the calf in two parts, and passed between the divisions of it, as well as the princes of Judah, and the nobles of Jerusalem, and the whole people of the land. Nevertheless, there appears to me to have been this special reason for the act referred to: that the Lord would indeed admonish the descendants of Abram, not only that they should be like a dead carcass, but even like one torn and dissected.

For the servitude with which they were oppressed for a time was more intolerable than simple death; yet because the sacrifice is offered to God, death itself is immediately turned into new life. And this is the reason why Abram, placing the parts of the sacrifice opposite to each other, fits them to each other, because they were to be gathered together again from their dispersion.

But how difficult the restoration of the Church is, and what troubles are involved in it, is shown by the horror with which Abram was seized. We see, therefore, that two things were illustrated: namely, the hard servitude with which the descendants of Abram were to be pressed almost to laceration and destruction; and then their redemption, which was to be the clear pledge of divine adoption. And in the same mirror, the general condition of the Church is represented to us, as it is the distinctive work of God to create it out of nothing and to raise it from death.