Objection 1. It would seem that without grace man cannot love God above all things by his own natural powers. For to love God above all things is the proper and principal act of charity. Now man cannot of himself possess charity, since the "charity of God is poured forth in our hearts by the Holy Ghost Who is given to us," as is said Romans 5:5. Therefore man by his natural powers alone cannot love God above all things.
Objection 2. Further, no nature can rise above itself. But to love God above all things is to tend above oneself. Therefore without the help of grace no created nature can love God above itself.
Objection 3. Further, to God, Who is the Highest Good, is due the best love, which is that He be loved above all things. Now without grace man is not capable of giving God the best love, which is His due; otherwise it would be useless to add grace. Hence man, without grace and with his natural powers alone, cannot love God above all things.
On the contrary, As some maintain, man was first made with only natural endowments; and in this state it is manifest that he loved God to some extent. But he did not love God equally with himself, or less than himself, otherwise he would have sinned. Therefore he loved God above himself. Therefore man, by his natural powers alone, can love God more than himself and above all things.
I answer that, As was said above (I, 60, 5), where the various opinions concerning the natural love of the angels were set forth, man in a state of perfect nature, could by his natural power, do the good natural to him without the addition of any gratuitous gift, though not without the help of God moving him. Now to love God above all things is natural to man and to every nature, not only rational but irrational, and even to inanimate nature according to the manner of love which can belong to each creature. And the reason of this is that it is natural to all to seek and love things according as they are naturally fit (to be sought and loved) since "all things act according as they are naturally fit" as stated in Phys. ii, 8. Now it is manifest that the good of the part is for the good of the whole; hence everything, by its natural appetite and love, loves its own proper good on account of the common good of the whole universe, which is God. Hence Dionysius says (Div. Nom. iv) that "God leads everything to love of Himself." Hence in the state of perfect nature man referred the love of himself and of all other things to the love of God as to its end; and thus he loved God more than himself and above all things. But in the state of corrupt nature man falls short of this in the appetite of his rational will, which, unless it is cured by God's grace, follows its private good, on account of the corruption of nature. And hence we must say that in the state of perfect nature man did not need the gift of grace added to his natural endowments, in order to love God above all things naturally, although he needed God's help to move him to it; but in the state of corrupt nature man needs, even for this, the help of grace to heal his nature.
Reply to Objection 1. Charity loves God above all things in a higher way than nature does. For nature loves God above all things inasmuch as He is the beginning and the end of natural good; whereas charity loves Him, as He is the object of beatitude, and inasmuch as man has a spiritual fellowship with God. Moreover charity adds to natural love of God a certain quickness and joy, in the same way that every habit of virtue adds to the good act which is done merely by the natural reason of a man who has not the habit of virtue.
Reply to Objection 2. When it is said that nature cannot rise above itself, we must not understand this as if it could not be drawn to any object above itself, for it is clear that our intellect by its natural knowledge can know things above itself, as is shown in our natural knowledge of God. But we are to understand that nature cannot rise to an act exceeding the proportion of its strength. Now to love God above all things is not such an act; for it is natural to every creature, as was said above.
Reply to Objection 3. Love is said to be best, both with respect to degree of love, and with regard to the motive of loving, and the mode of love. And thus the highest degree of love is that whereby charity loves God as the giver of beatitude, as was said above.
Source: Summa Theologica (New Advent)