Thou that wast tender, and didst use to melt under the word, under afflictions, wilt grow as some have profanely spoken, “sermon-proof and sickness-proof.” Thou that didst tremble at the presence of God, thoughts of death, and appearance before him, when thou hadst more assurance of his love than now thou hast, shalt have a stoutness upon thy spirit not to be moved by these things. Thy soul and thy sin shall be spoken of and spoken to, and thou shalt not be at all concerned, but shalt be able to pass over duties, praying, hearing, reading, and thy heart not in the least affected.
Sin will grow a light thing to thee; thou wilt pass it by as a thing of nought; this it will grow to. And what will be the end of such a condition? Can a sadder thing befall thee? Is it not enough to make any heart to tremble, to think of being brought into that estate wherein he should have slight thoughts of sin? Slight thoughts of grace, of mercy, of the blood of Christ, of the law, heaven, and hell, come all in at the same season. Take heed, this is that thy lust is working towards,—the hardening of the heart, searing of the conscience, blinding of the mind, stupifying of the affections, and deceiving of the whole soul.
– John Owen –
from The Mortification of Sin in Believers, volume 6 of Works, page 52