Moral, not metaphysical, infallibility
Thesis CXXXVIII. They add that efficacious grace is infallible in producing its effect not absolutely and metaphysically but rather morally. Being supremely congruous and proportioned to the will's disposition, it moves the will infallibly, yet in a very gentle way that agrees with the will's own temper.
It is like a man struggling with the direst want and poverty: physically he could turn down a hundred gold coins offered to him, but morally speaking, things being as they are, or given his state, situation and misfortune, he will infallibly accept them, because they are so well suited to relieving his distress.
Latin
CXXXVIII. Addunt gratiam efficacem in effectu suo producendo infallibilem esse, non absolute & Metaphysice, sed potius moraliter: quoniam cum sit voluntatis dispositioni maxime congrua & proportionata, voluntatem infallibiliter sed ratione admodum suavi, & voluntatis genio consentanea movet. Ad eum modum quo Homo summa egestate & inopia laborans, quamvis Physice possit centum aureos sibi oblatos respuere, tamen moraliter loquendo, positis rebus ut sunt, seu habita ratione status, conditionis, & calamitatis, eos infallibiliter acceptabit, quia sunt valde accommodati ad sublevandam illius miseriam.