De natura et gratia libri III, cum Apologia contra Catharinum

Similarly, concerning the gift of perseverance, of which it is written, He who perseveres to the end, he shall be saved (which indeed cannot be obtained from elsewhere, except from him who is able to establish him who stands so that he stands perseveringly, and to restore him who falls), let no one promise himself anything certain with absolute certainty. Although all ought to place and repose a most firm hope in the help of God. For God, unless they themselves fail his grace, just as he began the good work, so will he perfect it, working to will and to perfect. Nevertheless, let those who think they stand see that they do not fall, and with fear and trembling work out their salvation, in labours, in watchings, in almsdeeds, in prayers and oblations, in fastings, in chastity. For they ought to fear, knowing that they are born again into the hope of glory, and not yet into glory, concerning the struggle that remains with the flesh, with the world, with the devil, in which they cannot be victors unless with the grace of God they obey the Apostle, saying, 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.