Sin will grow a light thing to thee...can a sadder thing befall thee?

From "Mortification of Sin," Chapter 10:

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 tremble, to think of being brought into that estate wherein he should have slight thoughts of sin?