Thomas’ epistemology can be said to hold in tension the biblical doctrines of creation and salvation. He believed that humans are created in the image of God (imago Dei) and therefore have within them the capacity for true and rational thinking. The fall, of course, has obscured this thinking and it leads us to error and sin, but the indelible image of our Creator has not, according to Aquinas, fallen out of our minds. Yet, as Thomas also taught, salvation comes by grace, through Christ, to this same sinful humanity. The doctrine of salvation teaches us that we are not perfect and that our sin can easily obscure the truth. Thomas thus lands on a proposed solution to the problem that he believes will hold both of these truths together: grace perfects nature; it does not destroy it. In other words, though our minds are fallen, they are not destroyed; also, though our natural minds are sinful, yet they receive grace to grasp the truth. This solution was ingenious in that it drew philosophy and science together rather than forcing them apart.
Other areas of Thomas’ teachings are problematic. Take, for example, his discussion of sex. Thomas applies his paradigm of “grace perfecting nature” to the question of whether unnatural sins such as sodomy, masturbation, and fornication—indeed any sexual sins outside of marriage—are the greatest sins a person can commit. Thomas contends that these sins are an utter violation of the natural human process of procreation, and therefore constitute a violation of the natural created order as well as a rejection of the grace of Christ that blesses and restores the marriage covenant. Thus, Thomas argues, these sins are indeed among the worst sins possible. Not a few theologians have noted that Aquinas, along with other medieval scholars, ignored biblical teaching on the intimacy and pleasure of the marriage bed, and thereby paved the way for future Roman Catholic theologians to stress procreation as the essence of sex in marriage. It also, and more problematically, suggests an unbiblical gradation of sin by attempting to distinguish between “mortal” (serious) and “venial” (unintentional or small) sins. How can it be said, for example, that sexual sins are more sinful than sins of violence and rage? Why must sexual sins be considered the worst violation of God’s law when the fundamental root of all sin is self-sufficient pride?
Further problems are evident in Thomas’ teaching on justification. On this subject Aquinas stands tall among medieval theologians, though he later came under serious fire by Protestant theologians. Like many medieval theologians, Thomas taught that Christians receive an infusion of grace at baptism that remains within the soul, though it does not take over the will and force it to do good works. Aquinas holds to a doctrine of predestination, since God chooses by His own will who will receive the infusion of grace through baptism. Still, Thomas refused to conclude that good works are motivated by the Holy Spirit acting upon the will to inspire us to obedience. For Thomas, if love is to be authentic, it must be our own works of love in cooperation with grace. The ethical goal of the Christian life, then, is to actualize this infused grace through good works, which guide us in life unto salvation. These works of love are required of believers in order to receive eternal life, though Aquinas believes he avoids Pelagianism by stressing that the first step in the salvation process is God’s gift of grace apart from works.
The most fitting analogy of Aquinas’ teachings on salvation is that of exercise. We are all humans, but some of us are flabby and some of us are fit. We have all received our essence from God, since we are all rational beings created in His image. But in order to become more than flabby couch-dwellers, we must exercise our will through the effort of physical labor. No one can lift our arms and legs for us during exercise; the labor is our own. So, too, Thomas believed salvation was a matter of God infusing in the soul the grace sufficient to exercise in works of love, which then lead to eternal life. Our will must grasp this grace and exercise what Thomas called the “habits of grace” in order to grow in love towards Christ-likeness.
Thomas’s teachings on epistemology, justification, and ethics are among the most interesting and important subjects that continue to draw theologians to his many writings. Indeed, though Protestants have rejected not a few of his teachings since the Reformation, we can nevertheless look back—as did John Calvin, Philip Melanchthon, Martin Bucer, and even Luther in his quieter moments—and respect the heroic efforts of a theologian who has shaped our thinking for nearly eight hundred years.