Objection 1. It would seem that man can know that he has grace. For grace by its physical reality is in the soul. Now the soul has most certain knowledge of those things that are in it by their physical reality, as appears from Augustine (Gen. ad lit. xii, 31). Hence grace may be known most certainly by one who has grace.
Objection 2. Further, as knowledge is a gift of God, so is grace. But whoever receives knowledge from God, knows that he has knowledge, according to Wisdom 7:17: The Lord "hath given me the true knowledge of the things that are." Hence, with equal reason, whoever receives grace from God, knows that he has grace.
Objection 3. Further, light is more knowable than darkness, since, according to the Apostle (Ephesians 5:13), "all that is made manifest is light," Now sin, which is spiritual darkness, may be known with certainty by one that is in sin. Much more, therefore, may grace, which is spiritual light, be known.
Objection 4. Further, the Apostle says (1 Corinthians 2:12): "Now we have received not the Spirit of this world, but the Spirit that is of God; that we may know the things that are given us from God." Now grace is God's first gift. Hence, the man who receives grace by the Holy Spirit, by the same Holy Spirit knows the grace given to him.
Objection 5. Further, it was said by the Lord to Abraham (Genesis 22:12): "Now I know that thou fearest God," i.e. "I have made thee know." Now He is speaking there of chaste fear, which is not apart from grace. Hence a man may know that he has grace.
On the contrary, It is written (Ecclesiastes 9:1): "Man knoweth not whether he be worthy of love or hatred." Now sanctifying grace maketh a man worthy of God's love. Therefore no one can know whether he has sanctifying grace.
I answer that, 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."
Reply to Objection 1. Those things which are in the soul by their physical reality, are known through experimental knowledge; in so far as through acts man has experience of their inward principles: thus when we wish, we perceive that we have a will; and when we exercise the functions of life, we observe that there is life in us.
Reply to Objection 2. It is an essential condition of knowledge that a man should have certitude of the objects of knowledge; and again, it is an essential condition of faith that a man should be certain of the things of faith, and this, because certitude belongs to the perfection of the intellect, wherein these gifts exist. Hence, whoever has knowledge or faith is certain that he has them. But it is otherwise with grace and charity and such like, which perfect the appetitive faculty.
Reply to Objection 3. Sin has for its principal object commutable good, which is known to us. But the object or end of grace is unknown to us on account of the greatness of its light, according to 1 Timothy 6:16: "Who... inhabiteth light inaccessible."
Reply to Objection 4. The Apostle is here speaking of the gifts of glory, which have been given to us in hope, and these we know most certainly by faith, although we do not know for certain that we have grace to enable us to merit them. Or it may be said that he is speaking of the privileged knowledge, which comes of revelation. Hence he adds (1 Corinthians 2:10): "But to us God hath revealed them by His Spirit."
Reply to Objection 5. What was said to Abraham may refer to experimental knowledge which springs from deeds of which we are cognizant. For in the deed that Abraham had just wrought, he could know experimentally that he had the fear of God. Or it may refer to a revelation.
Source: Summa Theologica (New Advent)