10 It is plain then, my Brethren, it is plain to all, do ye hold it fast, and be sure of it, that whenever any one turns himself to the faith of our Lord Jesus Christ, from a useless or abandoned way of life, all that is past is forgiven him, and as though all his debts were cancelled, a new account is entered into with him. All is entirely forgiven. Let no one be anxious in the thought that there remains anything which is not forgiven him. But on the other hand, let no one rest in a perverse security.
For these two things are the death of souls, despair, and perverse hope. For as a good and right hope saves, so does a perverse hope deceive. First, consider how despair deceives. There are men, who when they begin to reflect on the evils they have done, think they cannot be forgiven; and while they think they cannot be forgiven, immediately they give up their souls to ruin, and perish through despair, saying in their thoughts, “Now there is no hope for us; for such great sins as we have committed cannot be remitted or pardoned us; why then should we not satisfy our lusts?
Let us at least fill up the pleasure of the time present, seeing we have no reward in that which is to come. Let us do what we list, though it be not lawful; that we may at least have a temporal enjoyment, because we cannot attain to the receiving an eternal.” In saying such things they perish through despair, either before they believe at all, or when Christians already, they have fallen by evil living into any sins and wickednesses. The Lord of the vineyard goes forth to them, and by the Prophet Ezekial knocks, and calls to them in their despair, and as they turn their backs to Him that calls them.
“In whatsoever day a man shall turn from his most wicked way, I will forget all his iniquities.” If they hear and believe this voice, they are recovered from despair, and rise up again from that very deep and bottomless gulf, wherein they had been sunk.
Source: Sermons on Selected Lessons of the New Testament (New Advent)