Objection 1. It would seem that man without grace, and by his own natural powers, can fulfil the commandments of the Law. For the Apostle says (Romans 2:14) that "the Gentiles who have not the law, do by nature those things that are of the Law." Now what a man does naturally he can do of himself without grace. Hence a man can fulfil the commandments of the Law without grace.
Objection 2. Further, Jerome says (Expos. Cathol. Fide [Symboli Explanatio ad Damasum, among the supposititious works of St. Jerome: now ascribed to Pelagius]) that "they are anathema who say God has laid impossibilities upon man." Now what a man cannot fulfil by himself is impossible to him. Therefore a man can fulfil all the commandments of himself.
Objection 3. Further, of all the commandments of the Law, the greatest is this, "Thou shalt love the Lord thy God with thy whole heart" (Matthew 27:37). Now man with his natural endowments can fulfil this command by loving God above all things, as stated above (Article 3). Therefore man can fulfil all the commandments of the Law without grace.
On the contrary, Augustine says (De Haeres. lxxxviii) that it is part of the Pelagian heresy that "they believe that without grace man can fulfil all the Divine commandments."
I answer that, There are two ways of fulfilling the commandments of the Law. The first regards the substance of the works, as when a man does works of justice, fortitude, and of other virtues. And in this way man in the state of perfect nature could fulfil all the commandments of the Law; otherwise he would have been unable to sin in that state, since to sin is nothing else than to transgress the Divine commandments. But in the state of corrupted nature man cannot fulfil all the Divine commandments without healing grace. Secondly, the commandments of the law can be fulfilled, not merely as regards the substance of the act, but also as regards the mode of acting, i.e. their being done out of charity. And in this way, neither in the state of perfect nature, nor in the state of corrupt nature can man fulfil the commandments of the law without grace. Hence, Augustine (De Corrupt. et Grat. ii) having stated that "without grace men can do no good whatever," adds: "Not only do they know by its light what to do, but by its help they do lovingly what they know." Beyond this, in both states they need the help of God's motion in order to fulfil the commandments, as stated above (2,3).
Reply to Objection 1. As Augustine says (De Spir. et Lit. xxvii), "do not be disturbed at his saying that they do by nature those things that are of the Law; for the Spirit of grace works this, in order to restore in us the image of God, after which we were naturally made."
Reply to Objection 2. What we can do with the Divine assistance is not altogether impossible to us; according to the Philosopher (Ethic. iii, 3): "What we can do through our friends, we can do, in some sense, by ourselves." Hence Jerome [Symboli Explanatio ad Damasum, among the supposititious works of St. Jerome: now ascribed to Pelagius] concedes that "our will is in such a way free that we must confess we still require God's help."
Reply to Objection 3. Man cannot, with his purely natural endowments, fulfil the precept of the love of God, as stated above (Article 3).
Source: Summa Theologica (New Advent)