Commenting on the hardening of Pharaoh’s heart in Exo. 4:21 Luther paraphrases it this way:
I will certainly deliver you, but you will find it hard to believe, because Pharaoh will so resist and delay the deliverance. But trust nevertheless; for by My operation all his delaying shall only result in My performing more and greater miracles to confirm you in your faith and to display My powers, so that henceforth you may have more faith in Me in all other matters.
Amen and amen. This has been our journey, one of delay in which our faith has been confirmed and God’s power displayed.
From The Bondage of the Will. Translated by J. I. Packer and O. R. Johnston, 210.