Ephesians 4:18

Being darkened in their understanding, alienated from the life of God, because of the ignorance that is in them, because of the hardening of their heart; who being past feeling gave themselves up to lasciviousness, to work all uncleanness with greediness.

Here is a reference to the pre-Christian Gentiles who at first knew God, rebelled against him, turned away from him, eventually being hardened, first through their own wickedness, and later receiving a judicial hardening of God himself who thus punished their disobedience. See extended discussion of that deplorable condition of the pre-Christian Gentiles in my Commentary on Romans 1:21ff. These two verses are a thumbnail reference to a matter Paul discussed at length in Romans, chapter 1.

Hardening of their heart ... gave themselves up ... See special related articles on these topics: "When God Gives up on People" (my Commentary on Romans 1:28), and "The Hardening of Israel" in my Commentary on Romans 11.