68 We see the same thing in the Gospel, where the devils confess that Christ is the Son of God in the words used by Peter, but with a very different heart. So, though the words were the same, Peter is praised for his faith, while the impiety of the devils is checked. For Christ, not by human sense, but by divine knowledge, could inspect and infallibly discriminate the sources from which the words came. Besides, there are multitudes who confess that Christ is the Son of the living God, without meriting the same approval as Peter— not only of those who shall say in that day, "Lord, Lord," and shall receive the sentence, "Depart from me," but also of those who shall be placed on the right hand.
They may probably never have denied Christ even once; they may never have opposed His suffering for our salvation; they may never have forced the Gentiles to do as the Jews; and yet they shall not be honored equally with Peter, who, though he did all these things, will sit on one of the twelve thrones, and judge not only the twelve tribes, but the angels. So, again, many who have never desired another man's wife, or procured the death of the husband, as David did, will never reach the place which David nevertheless held in the divine favor.
There is a vast difference between what is in itself so undesirable that it must be utterly rejected, and the rich and plenteous harvest which may afterwards appear. For farmers are best pleased with the fields from which, after weeding them, it may be, of great thistles, they receive an hundred-fold; not with fields which have never had any thistles, and hardly bear thirty-fold.
Source: Reply to Faustus the Manichaean (New Advent)