Council of Trent: Session 6
Chapter 12
That a rash presumptuousness in the matter of Predestination is to be avoided.
No one, moreover, so long as he is in this mortal life, ought so far to presume as regards the secret mystery of divine predestination, as to determine for certain that he is assuredly in the number of the predestinate; as if it were true, that he that is justified, either cannot sin any more, or, if he do sin, that he ought to promise himself an assured repentance; for except by special revelation, it cannot be known whom God hath chosen unto Himself.
Chapter 13
On the gift of Perseverance.
So also as regards the gift of perseverance, of which it is written, He that shall persevere to the end, he shall be saved:—which gift cannot be derived from any other but Him, who is able to establish him who standeth that he stand perseveringly, and to restore him who falleth:—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. Nevertheless, let those who think themselves to stand, take heed lest they fall, and, with fear and trembling work out their salvation, in labours, in watchings, in almsdeeds, in prayers and oblations, in fastings and chastity: for, knowing that they are born again unto a hope of glory, but not as yet unto glory, they ought to fear for the combat which yet remains with the flesh, with the world, with the devil, wherein they cannot be victorious, unless they be with God's grace, obedient to the Apostle, who says; We are debtors, not to the flesh, to live according to the flesh; for if you live according to the flesh, you shall die; but if by the spirit you mortify the deeds of the flesh, you shall live.
Chapter 14
On the fallen, and their restoration.
As regards those who, by sin, have fallen from the received grace of Justification, they may be again justified, when, God exciting them, through the sacrament of Penance they shall have attained to the recovery, by the merit of Christ, of the grace lost: for this manner of Justification is of the fallen the reparation: which the holy Fathers have aptly called a second plank after the shipwreck of grace lost. For, on behalf of those who fall into sins after baptism, Christ Jesus instituted the sacrament of Penance, when He said, Receive ye the Holy Ghost, whose sins you shall forgive, they are forgiven them, and whose sins you shall retain, they are retained. Whence it is to be taught, that the penitence of a Christian, after his fall, is very different from that at (his) baptism; and that therein are included not only a cessation from sins, and a detestation thereof, or, a contrite and humble heart, but also the sacramental confession of the said sins,—at least in desire, and to be made in its season,—and sacerdotal absolution; and likewise satisfaction by fasts, alms, prayers, and the other pious exercises of a spiritual life; not indeed for the eternal punishment,—which is, together with the guilt, remitted, either by the sacrament, or by the desire of the sacrament,—but for the temporal punishment, which, as the sacred writings teach, is not always wholly remitted, as is done in baptism, to those who, ungrateful to the grace of God which they have received, have grieved the Holy Spirit, and have not feared to violate the temple of God. Concerning which penitence it is written; Be mindful whence thou art fallen; do penance, and do the first works. And again; The sorrow that is according to God worketh penance steadfast unto salvation. And again; Do penance, and bring forth fruits worthy of penance.