“I am not aware of anything against myself, but I am not thereby acquitted. It is the Lord who judges me. Therefore do not pronounce judgment before the time, before the Lord comes, who will bring to light the things now hidden in darkness and will disclose the purposes of the heart.”
Short answer
The Church teaches that no believer should doubt God's mercy, the merit of Christ or the truth of his promises, and that all should place a most firm hope in God's help. She denies one thing: that a person can know with the certainty of faith, apart from a special revelation, that he is in grace or that he will persevere to the end. The certainty of Christian hope rests on God, not on a reading of one's own soul.
Drafted from primary sources. Every quotation is checked against the page it links to.
16 quotations in date order
“For in this hope we were saved. Now hope that is seen is not hope. For who hopes for what he sees? But if we hope for what we do not see, we wait for it with patience.”
“And we desire each one of you to show the same earnestness in realizing the full assurance of hope until the end, so that you may not be sluggish, but imitators of those who through faith and patience inherit the promises.”
“And by this we may be sure that we know him, if we keep his commandments. He who says “I know him” but disobeys his commandments is a liar, and the truth is not in him;”
“We ought indeed to walk so holily, and with so entire substantiality of faith, as to be confident and secure in regard of our own conscience, desiring that that (gift) may abide in us to the end, yet not presuming (that it will). For he who presumes feels less apprehension; he who feels less apprehension takes less precaution; he who takes less precaution runs more risk.”
“If a grave and praiseworthy man should promise you anything, you would assuredly have faith in the promiser, and would not think that you should be cheated and deceived by him whom you knew to be steadfast in his words and his deeds. Now God is speaking with you; and do you faithlessly waver in your unbelieving mind? God promises to you, on your departure from this world, immortality and eternity; and do you doubt?”
“And how could it be, if he knew nothing against himself that he was not justified? Because it was possible for him to have committed certain sins, not however, knowing that they were sins.”
“These, though they are certain that they shall be rewarded if they persevere, are not certain that they will persevere. For what man can know that he will persevere to the end in the exercise and increase of grace, unless he has been certified by some revelation from Him who, in His just and secret judgment, while He deceives none, informs few regarding this matter?”
“…I know that you fervently love the Almighty Lord, and I trust in His mercy that the sentence which was pronounced with regard to a certain holy woman proceeds from the mouth of the Truth with regard to you: Her sins, which are many, are forgiven her, for she loved much … you have demanded a difficult, nay even an unprofitable thing; difficult indeed, because I am unworthy of having a revelation made to me; but unprofitable, because you ought not to become secure about your sins, except when in the last day of your life you shall be able no longer to bewail them.”
“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, … Secondly, a man may, of himself, know something, and with certainty; and in this way no one can know that he has grace. … 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.”
“Hope does not trust chiefly in grace already received, but on God's omnipotence and mercy, whereby even he that has not grace, can obtain it, so as to come to eternal life. Now whoever has faith is certain of God's omnipotence and mercy.”
“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 cannot be subject to error, that he has obtained the grace of God.”
“…let no one herein promise himself any thing as certain with an absolute certainty; though all ought to place and repose a most firm hope in God's help. For God, unless men be themselves wanting to His grace, as he has begun the good work, so will he perfect it, working (in them) to will and to accomplish.”
“Since it belongs to the supernatural order, grace escapes our experience and cannot be known except by faith. We cannot therefore rely on our feelings or our works to conclude that we are justified and saved. … "Asked if she knew that she was in God's grace, she replied: 'If I am not, may it please God to put me in it; if I am, may it please God to keep me there.'"”
“We can therefore hope in the glory of heaven promised by God to those who love him and do his will. In every circumstance, each one of us should hope, with the grace of God, to persevere "to the end" and to obtain the joy of heaven, as God's eternal reward for the good works accomplished with the grace of Christ.”
“Redemption is offered to us in the sense that we have been given hope, trustworthy hope, by virtue of which we can face our present: the present, even if it is arduous, can be lived and accepted if it leads towards a goal, if we can be sure of this goal, and if this goal is great enough to justify the effort of the journey.”