For with God nothing shall be impossible.
God's word and His power are not two different things, to promise is already to accomplish. We waste our breath imagining what God could do apart from what He resolves to do; the only profitable doctrine is that nothing resists His will. Mary needed reminding because we all do: we confess God almighty in theory, then doubt the moment He says something beyond our grasp.
AI summary
We have here notice given us of all that it was fit we should know concerning the incarnation and conception of our blessed Saviour, six months after the conception of John. The same angel, Gabriel, that was employed in making known to Zacharias God's purpose concerning his son, is employed in this also; for in this, the same glorious work of redemption, which was begun in that, is carried on.
Commenting on Luke 1:26-38
Every word God has spoken, every promise made, every prediction uttered by His prophets, He is both able and willing to perform. This virgin bearing a son is no exception; the angel confirms Mary's faith by pointing her to the very omnipotence that makes the impossible certain.
AI summary