The fullest ability comes only from actual grace
Thesis XV. He insists, though, that we should not be said to be able in the fullest sense to do good and carry out the duties of devotion except when the Holy Spirit's inspiration so prepares the will that it does not merely have the power but actually wills. On his teaching, then, this fullest ability is had only when the deed is actually done as well. It is the proper effect not of faith or love, that is, of a habitually good will, but of the actual grace that Christ brought to weak humanity by his cross. That grace assists not only the ability but the doing itself, since it gives not merely 'the ability, if you will' but also 'the will to do what you can'.
So however great a person's faith and love, or habitually good will, may be, even of the strongest kind, if a temptation arises, however slight, and he is not fortified by such an inspiration of the Spirit, that is, by actual grace, he will certainly give way, because without it, since the first man's fall, the will or habitual love cannot pass into action. For the tinder of inborn concupiscence constantly presses the listless soul downward, and is itself constantly stirred up by visible objects presenting themselves from every side, inside and out, unless the gift of the Holy Spirit either heads off its stirrings before they rise or crushes them once they have risen.
Latin
XV. Sed pertendit nos dici non debere completissime posse bene facere, & pietatis officiis fungi, nisi quando sancti spiritus inspiratione sic voluntas praeparatur, ut non nude possit, sed etiam velit. Adeoque, ex ipsius doctrina, hoc completissimum posse nunquam habetur, nisi quando reipsa quoque agitur, & est proprius effectus non fidei aut charitatis, seu bonae voluntatis habitualis, sed illius gratiae actualis, quam Christus hominibus infirmis attulit per crucem suam. Qua gratia non solum posse, sed etiam ipsum agere adjuvatur: ut quae dat non modo posse si velis, sed & velle quod potes. Ac proinde quantacunque fide, & charitate, seu habituali bona voluntate praeditus aliquis fuerit, etiam robustissima, nisi tentatione, quamvis levi, oboriente, tali spiritus inspiratione, seu actuali gratia muniatur, sine dubitatione succumbet: quia sine illa, voluntas seu charitas habitualis post lapsum primi hominis in actum exire non potest. Propterea quod innatae concupiscentiae fomes semper animum languentem deorsum premit; & semper visibilibus objectis undiquaque intus & foris sese offerentibus proritatur, nisi spiritus sancti munere motus ejus vel praeveniantur orituri, vel comprimantur exorti.