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[…]. 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 cannot be subject to error, that he has obtained the grace of God.

–Council of Trent, Session 6, Chapter IX

And whereas the Apostle saith, that man is justified by faith and freely, those words are to be understood in that sense which the perpetual consent of the Catholic Church hath held and expressed; to wit, that we are therefore said to be justified by faith, because faith is the beginning of human salvation, the foundation, and the root of all Justification; without which it is impossible to please God, and to come unto the fellowship of His sons: but we are therefore said to be justified freely, because that none of those things which precede justification–whether faith or works–merit the grace itself of justification. For, if it be a grace, it is not now by works, otherwise, as the same Apostle says, grace is no more grace.

–Council of Trent, Session 6, Chapter VIII