It is proper... to confess our sins, because there may be sins in us, and not merely those which result from infirmity and are involuntary, which are seen by the omniscient eye of God, but which may not be obvious to ourselves. We have no doubt that, as a general thing, we may rely upon our consciousness in confirmation of the great fact of perfection in love. Certainly it is a reasonable idea, that, as a general thing, a man may know in himself or in his own consciousness, whether he loves God or not; and whether he loves him with his whole heart or not. At the same time there may occasionally be cases, in which he is left in some degree of doubt. He may through the influence of some sudden temptation, be driven so closely upon the line which separates rectitude from sin, that it is almost impossible for him to tell whether he has kept within it. The Scriptures also recognize the great deceitfulness of the human heart. Who, then, is able, either on philosophical or scripture principles, to assert, absolutely and unconditionally, that he has been free from sin, at least for any great length of time? We may, therefore, with great propriety, even if there were no other reason but this, ask the forgiveness of our trespasses, of our sins, or of whatever God sees amiss in us. And it is unquestionably our duty so to do.
We may add here, that it is generally, and perhaps we may say, universally the case that those, who give good evidence of being in that state which we variously describe as assurance of faith and as perfect love, and which involves the possession of the blessing of present sanctification, speak of their state in a qualified, rather than in an absolute manner. In other words, they generally express themselves, (and it is exceedingly proper that they should do so,) merely as if they hoped or had reason to hope that they had experienced this great blessing, and were kept free from voluntary and known sin. Such a mode of expression seems to be unobjectionable; it is consistent with confession, and corresponds to the precise state of the case.
— edited from The Interior or Hidden Life (2nd edition, 1844) Part 2, Chapter 16 by Thomas Cogswell Upham. You can find more of his work at the blog, The Hidden Life, managed by Craig L Adams at: http://thomascupham.blogspot.com