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There are three ways of knowing a thing: first, by revelation, and thus anyone may know that he has grace, for God by a special privilege reveals this at times to some, in order that the joy of safety may begin in them even in this life, and that they may carry on toilsome works with greater trust and greater energy, and may bear the evils of this present life, as when it was said to Paul (2 Corinthians 12:9): "My grace is sufficient for thee."
Secondly, a man may, of himself, know something, and with certainty; and in this way no one can know that he has grace. For certitude about a thing can only be had when we may judge of it by its proper principle. Thus it is by undemonstrable universal principles that certitude is obtained concerning demonstrative conclusions. Now no one can know he has the knowledge of a conclusion if he does not know its principle. But the principle of grace and its object is God, Who by reason of His very excellence is unknown to us, according to Job 36:26: "Behold God is great, exceeding our knowledge." And hence His presence in us and His absence cannot be known with certainty, according to Job 9:11: "If He come to me, I shall not see Him; if He depart I shall not understand." And hence man cannot judge with certainty that he has grace, according to 1 Corinthians 4:3-4: "But neither do I judge my own self . . . but He that judgeth me is the Lord."
Thirdly, things are known conjecturally by signs; and thus anyone may know he has grace, when he is conscious of delighting in God, and of despising worldly things, and inasmuch as a man is not conscious of any mortal sin. And thus it is written (Apocalypse 2:17): "To him that overcometh I will give the hidden manna . . . which no man knoweth, but he that receiveth it," because whoever receives it knows, by experiencing a certain sweetness, which he who does not receive it, does not experience. Yet this knowledge is imperfect; hence the Apostle says (1 Corinthians 4:4): "I am not conscious to myself of anything, yet am I not hereby justified," since, according to Psalm 18:13: "Who can understand sins? From my secret ones cleanse me, O Lord, and from those of others spare Thy servant."
ST I-II. Q. 112, A. 5.