De natura et gratia libri III, cum Apologia contra Catharinum

But although it is necessary to believe that sins are not forgiven, nor have ever been forgiven, except freely by divine mercy on account of Christ, nevertheless it must not be said that no one is forgiven or has been forgiven who boasts of confidence and certainty of the remission of his sins and rests in that alone, since among heretics and schismatics this vain confidence, far removed from all piety, can exist, and indeed in our own time does exist and is preached with great contention against the Catholic Church; but neither must it be asserted that those who have been truly justified ought to establish with themselves, without any doubt at all, that they are justified, and that no one is absolved from sins and justified unless he believes with certainty and most firmly that he is absolved and justified, and that absolution and justification are accomplished by this faith alone, as if one who does not believe this doubts the promises of God and the efficacy of the death and resurrection of Christ. For as no pious person ought to doubt the mercy of God, the merit of Christ, or the virtue and efficacy of the sacraments, so anyone, when he looks at himself and his own weakness and indisposition, can fear and be afraid about his own grace, since no one can know with the certainty of faith, to which falsity cannot be subject, that he has obtained the grace of God.