John Calvin Commentary Genesis 16:10

"And the angel of Jehovah said unto her, I will greatly multiply thy seed, that it shall not be numbered for multitude." — Genesis 16:10 (ASV)

I will multiply your seed exceedingly. For the purpose of mitigating the offense and alleviating what was severe in the precept, he offers consolation by promising a blessing in the child she was to bear. God could indeed, by his own authority, have strictly commanded what was right; but so that Hagar might more cheerfully do what she knew to be her duty, he allures her to obedience with gentle persuasion.

And this is the purpose of those promises: through them, he invites us to voluntary submission. For he does not wish to draw us by servile methods, compelling us to obey his commands by constraint; and therefore, he mingles mild and paternal invitations with his commands, dealing with us generously, as with sons.

The fact that the angel here promises to do what belongs exclusively to God involves no absurdity. This is because it is quite common for God to endow the ministers he sends with his own authority, so that the authority of their word may appear greater. However, I do not disagree with the opinion of most of the ancient writers: that Christ the Mediator was always present in all divine revelations, and that this is why the majesty of God is ascribed to angels. I have already touched upon this subject and will have occasion to say more about it elsewhere.