Can a Christian in a state of grace know that they are?

Can a Christian in a state of grace know that they are?

The short answer is no.

This reminds me of a similar question posed to St. Joan of Arc in 1431, during Her trial by the English.

Third session: Saturday, February 24, 1431

Question:Do you know whether or not you are in God's grace?

Joan:If I am not, may God put me there; and if I am, may God so keep me. I should be the saddest creature in the world if I knew I were not in His grace."

The question was a deliberate attempt to entrap her, since the Church's doctrine held that no one could be certain of being in God's grace; and yet answering 'no' could also be used against her because the judge could claim she had admitted to being in a state of sin. According to the eyewitnesses, this question elicited a protest from one of the assessors, Jean Lefèvre, who said it was a "grave question" that Joan wasn't required to answer. Cauchon retorted: "It would have been better for you if you had kept your mouth shut!" Joan's response, neatly avoiding the theological trap, left the court "stupefied" according to one of the notaries, Boisguillaume. She added that if she were in a state of sin, she didn't think these saints would come to her; and she wished everyone could hear them as well as she did. She thought she was about thirteen years old when they came to her for the first time.

One can also listen to this YouTube video hosted by Matt Fradd (Pints with Aquinas): Can I know for sure whether I'm in the state of grace?.

To the question of "Whether man can know that he has grace?" (Summa Theologica I-II q. 112 a. 5), St. Thomas Aquinas responds by explaining the ways the reality of grace can or cannot be known:

There are three ways of knowing a thing:

1. 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 Cor. 12:9): "My grace is sufficient for thee."

2. 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 Cor. 4:3,4: "But neither do I judge my own self … but He that judgeth me is the Lord."

3. 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 (Apoc. 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 Cor. 4:4): "I am not conscious to myself of anything, yet am I not hereby justified," since, according to Ps. 18:13: "Who can understand sins? From my secret ones cleanse me, O Lord, and from those of others spare Thy servant."

^{source: this answer to the question "What are the practical effects of sanctifying grace?"}

Council of Trent session 6 on Justification, Chapter IX. "Against the vain confidence of heretics" (Denzinger 802):

But, although it is necessary to believe that sins neither are remitted, nor ever were remitted save gratuitously by the mercy of God for Christ's sake; yet is it not to be said, that sins are forgiven, or have been forgiven, to any one who boasts of his confidence and certainty of the remission of his sins, and rests on that alone; seeing that it may exist, yea does in our day exist, amongst heretics and schismatics; and with great vehemence is this vain confidence, and one alien from all godliness, preached up in opposition to the Catholic Church. But neither is this to be asserted—that they who are truly justified must needs, without any doubting whatever, settle within themselves that they are justified, and that no one is absolved from sins and justified, but he that believes for certain that he is absolved and justified; and that absolution and justification are effected by this faith alone: as though whoso has not this belief, doubts of the promises of God, and of the efficacy of the death and resurrection of Christ. For even as no pious person ought to doubt of the mercy of God, of the merit of Christ, and of the virtue and efficacy of the sacraments, even so each one, when he regards himself, and his own weakness and indisposition, may have fear and apprehension touching his own grace; seeing that no one can know with a certainty of faith, which can not be subject to error, that he has obtained the grace of God.

Trent also defined these de fide dogmas:

CANON XIII.—If any one saith, that it is necessary for every one, for the obtaining the remission of sins, that he believe for certain, and without any wavering arising from his own infirmity and indisposition, that his sins are forgiven him: let him be anathema.

CANON XVI.—If any one saith, that he will for certain, of an absolute and infallible certainty, have that great gift of perseverance unto the end,—unless he have learned this by special revelation: let him be anathema.