Hardness of heart is not a distinct sin, but the habitual power of every sin, or the deadness, unmovableness, and obstinacy of the heart in any sin…It is therefore an error, that hath had very ill consequences on many persons, to think that hardness of heart is nothing but a want of passionate feeling in the matters which concern the soul; especially a want of sorrow and tears. This hath made them over-careful for such tears, and grief, and passions, and dangerously to make light of the many greater instances of the hardness of their hearts…I entreat you therefore to observe, that the greater the duty is, the worse it is to harden the heart against it; and the greater the sin is, the worse it is to harden the heart by obstinacy in it. And that the great duties are, the love of God and man, with a mortified and heavenly mind and life; and to resist God’s word commanding these, is the great and dengerous hardening of the heart…The work of repentance consisteth most in loathing and falling out with ourselves for our sins, and in forsaking them with abhorrence, and turning unto God; and he that can do this without tears is truly penitent, and he that hath never so many tears, without this, is impenitent still. And that is the hard-hearted sinner, that will not be wrought to a love of holiness, nor let go his sin, when God commandeth him; but after all exhortations, and mercies, and perhaps afflictions, is still the same as if he had never been admonished, or took no notice what God hath been saying or doing to reclaim him.

-Richard Baxter, The Practical Works of Richard Baxter, Vol. I, pp. 171-2.