John Calvin on Genesis 15:11

11. And when the fowls came down . Although the sacrifice was dedicated to God, yet it was not free from the attack and the violence of birds. So neither are the faithful, after they are received into the protection of God, so covered with his hand, as not to be assailed on every side; since Satan and the world cease not to cause them trouble. Therefore, in order that the sacrifice we have once offered to God may not be violated, but may remain pure and uninjured, contrary assaults must be repulsed, with whatever inconvenience and toil.