Note 3 at Mr 8:17: Just a few days before this instance, hardened hearts had made Jesus' disciples amazed when they saw Jesus walk on the water to them (see note 10 at Mr 6:52). Here, Jesus' statements revealed some of the characteristics of someone with a hardened heart.

First, hard hearts keep us from perceiving spiritual truths. This is why everyone in a church service will hear exactly the same message, and some will receive while others won't. It's not the Word that's the variable but rather the condition of the hearts (see note 2 at Lu 8:11). Hard hearts stop spiritual perception (Ex 7:10-13, 19-23; 8:8-19, 31-32; 9:5-7, 10-12, 22-35; 10:13-27; 11:10; and 14:3-4).

Second, hard hearts stop us from understanding spiritual truths (1Co 2:14). When we don't understand God's Word, Satan finds no resistance when he comes to steal it away (Mt 13:19, see note 3 at Lu 8:12).

Third, hard hearts keep us from remembering. This isn't to say that we can't recall facts or Scriptures. As related in Mr 8:19-20, these disciples remembered the facts of the two miraculous feedings, but they had forgotten any spiritual lessons they may have learned. Likewise, some of us can quote Scripture or remember what the sermon was about, but we can't perceive the spiritual life in them or retain what we did perceive, because of hardened hearts.

Also, in Job 39:13-17, God spoke of the ostrich as being hardened toward her young and equated that with being deprived of wisdom. A hardened heart toward God produces spiritual retardation.