In Exodus, Moses is trying to persuade the ruler of Egypt, the Pharaoh, to let the enslaved Israelites go and worship God. But God, who is committed to making this happen, is said to “harden” the Pharaoh’s heart. God tells Moses in Exodus:

When you return to Egypt, see that you perform before Pharaoh all the wonders I have given you the power to do. But I will harden his heart so that he will not let the people go.

Some might interpret this as suggesting that God, in his desire to glorify himself, makes the Pharaoh sin (because that makes more of a spectacle). But I think that would create a rather strange picture of God. He cannot be responsible for sin; he has to be blameless because he is perfect. As Deuteronomy 32:4 (NIV) says:

He is the Rock, his works are perfect,
and all his ways are just.
A faithful God who does no wrong,
upright and just is he.

So we somehow need to interpret the idea of hardening a heart while also attributing sin to the Pharaoh.

It is worth noting that Moses does not simply attribute the hardening to God: Exodus says that God hardens the Pharaoh, but it also says that the Pharaoh hardens his own heart and, passively, that the Pharaoh’s heart was hardened. This suggests some complexity!

There is perhaps a connection between what’s happening with the Pharaoh and the behind-the-scenes work Christians believe God is doing with grace. Calvinist and Arminian theologies argue that humans are not able to turn to God on their own because they are enslaved by their sin. Instead, God’s grace gives people a helping hand.

So could it be that the natural state of affairs is a hardened heart and that what God is doing is simply leaving the Pharaoh to his (un)natural state?

That does seem to be a traditional take from commentaries. As John Wesley said:

And he harden’d Pharaoh’s heart – That is, permitted it to be hardened.

Matthew Henry’s 18th century commentary said of the Pharaoh:

Before, he had hardened his own heart, and resisted the grace of God; and now God justly gave him up to his own heart’s lusts, to a reprobate mind, and strong delusions, permitting Satan to blind and harden him, and ordering every thing, henceforward, so as to make him more and more obstinate. Note, Wilful hardness is commonly punished with judicial hardness. If men shut their eyes against the light, it is just with God to close their eyes.

And Thomas Aquinas, the 13th century theologian, wrote:

All these passages [on hardening the Pharaoh’s heart] are to be understood as meaning that God does not bestow on some the help for avoiding sin which He bestows on others. This help is not merely the infusion of grace, but also an exterior guardianship, whereby the occasions of sin are providentially removed from a man’s path. God also aids man against sin by the natural light of reason, and other natural goods that He bestows on man. When then He withdraws these aids from some, as their conduct deserves that he should, according to the exigency of His justice, He is said to harden them, or to blind them.