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An "uncircumcised heart" is one that is closed and impervious to God's attempts to affect it. It resists them, which is why Stephen calls them "stiff-necked." A stiff-necked person is unyielding. His head is set, his jaw is outthrust, his ears are closed, and his teeth are clenched. He says, "I won't do it!" This is the effect of the uncircumcised heart.
"Uncircumcised ears" are those that hear the Word of God imperfectly, usually because they hear only what they want to hear, or they hear with such a strong prejudice that they reject the truth out of hand. Interestingly, if God says something, it is likely that men will reject it, yet if a man says exactly the same thing, a high likelihood exists that the listener's mind will be much more open to what is said. This just shows how physically oriented we are. If we know something is coming from God, human nature always gets its guard up; it is already beginning to say, "No."
"Uncircumcised lips" (see Exodus 6:12, 30) speak the Word of God imperfectly or incorrectly, either because the person is in ignorance or has been deceived.
In regard to an uncircumcised heart, if what hinders a person from yielding to God is cut away—circumcised—the heart becomes open, pliable, and amenable to the Word of God. The effect is that he will submit.
John W. Ritenbaugh
The Covenants, Grace, and Law (Part 7)