“Make the heart.” An important thing to keep in mind when reading verses that say things such as God hardened someone’s heart, or covered their ears, or blinded their eyes, is that the people actually did those things to themselves: God does not harden someone’s heart and then punish him for having a hardened heart. That would be totally against the loving nature of God. Verses that say things like God hardened someone’s heart are written using standard Hebrew language and customs, and are using the Semitic “idiom of permission.”
The “idiom of permission” is a name given by scholars to the particular Semitic manner of speaking in which someone is said to actively do what he only allowed to be done. For example, God did not reach into Pharaoh’s heart and harden it, but He allowed Pharaoh to harden his own heart (actually, Pharaoh had free will and God could not stop him from hardening his heart). God asked Pharaoh over and over again to let His people go. Pharaoh refused. The more God asked, and the stronger God’s plague-warnings became, the more stubborn Pharaoh became and the harder he had to make his heart to resist God. So God was only hardening Pharaoh’s heart in the sense that Pharaoh had to harden his heart harder and harder to resist God’s appeals.
The same is true of the Jews. Through the centuries God sent prophet after prophet to Israel, and Israel suffered consequence after consequence because of their unbelief. The more prophets God sent, the more Israel hardened itself against them. So in the Semitic idiom, God is said to have hardened Israel’s heart, closed her ears, and blinded her eyes. [For more on the idiom of permission, see commentary on Rom. 9:18].
“fat.” Here, “fat” is an idiom for insensitive, unreceptive. dull. Israel was “fat” when it came to the things of God. They were insensitive and unreceptive.