Thomas Aquinas explains that grace is necessary for every good act.
I answer that, To know truth is a use or act of intellectual light, since, according to the Apostle (Eph. 5:13): “All that is made manifest is light.” Now every use implies movement, taking movement broadly, so as to call thinking and willing movements, as is clear from the Philosopher (De Anima iii, 4). Now in corporeal things we see that for movement there is required not merely the form which is the principle of the movement or action, but there is also required the motion of the first mover. Now the first mover in the order of corporeal things is the heavenly body. Hence no matter how perfectly fire has heat, it would not bring about alteration, except by the motion of the heavenly body. But it is clear that as all corporeal movements are reduced to the motion of the heavenly body as to the first corporeal mover, so all movements, both corporeal and spiritual, are reduced to the simple First Mover, Who is God. And hence no matter how perfect a corporeal or spiritual nature is supposed to be, it cannot proceed to its act unless it be moved by God; but this motion is according to the plan of His providence, and not by necessity of nature, as the motion of the heavenly body. Now not only is every motion from God as from the First Mover, but all formal perfection is from Him as from the First Act. And thus the act of the intellect or of any created being whatsoever depends upon God in two ways: first, inasmuch as it is from Him that it has the form whereby it acts; secondly, inasmuch as it is moved by Him to act.
And later,
Man’s nature may be looked at in two ways: first, in its integrity, as it was in our first parent before sin; secondly, as it is corrupted in us after the sin of our first parent. Now in both states human nature needs the help of God as First Mover, to do or wish any good whatsoever, as stated above (Article [1]). But in the state of integrity, as regards the sufficiency of the operative power, man by his natural endowments could wish and do the good proportionate to his nature, such as the good of acquired virtue; but not surpassing good, as the good of infused virtue. But in the state of corrupt nature, man falls short of what he could do by his nature, so that he is unable to fulfil it by his own natural powers. Yet because human nature is not altogether corrupted by sin, so as to be shorn of every natural good, even in the state of corrupted nature it can, by virtue of its natural endowments, work some particular good, as to build dwellings, plant vineyards, and the like; yet it cannot do all the good natural to it, so as to fall short in nothing; just as a sick man can of himself make some movements, yet he cannot be perfectly moved with the movements of one in health, unless by the help of medicine he be cured.
And thus in the state of perfect nature man needs a gratuitous strength superadded to natural strength for one reason, viz. in order to do and wish supernatural good; but for two reasons, in the state of corrupt nature, viz. in order to be healed, and furthermore in order to carry out works of supernatural virtue, which are meritorious. Beyond this, in both states man needs the Divine help, that he may be moved to act well.
dhspriory.org/thomas/summa/FS/FS109.html#FSQ109OUTP1
