Augustine and Aquinas, the medieval philosophers selected for our text, were what might be called "Hellenized Christians." They took the basic message of Christ, that the greatest commandment is to love God consummately and to love one's neighbor as one's self, and gave it the trappings of Greek philosophy. In general, they emphasized the subordination of the moral virtues to something higher, the highest good. The highest good, in turn, is identified with God in one way or another.
Augustine's message was more Platonic than Aristotelian. Like Plato, he rejected the world of the sensual as beneath our true nature. The flesh tempts us with pleasures which detract utterly from the divine calling. The highest good is to be united with God, with pure spirituality. Since our earthly world can give us no guidance in spiritual matters, we must "walk in faith." In particular, Augustine tried to justify the authority of the Catholic Church in instructing us about God.
It took some ingenuity for Augustine to work Jesus' message of love into his Platonic metaphysics. He tried to reduce each of the four virtues to love, in a way that I find overly intellectualized. For example, "prudence is love making a right distinction between what helps it towards God and what might hinder it." Another strained piece of reasoning is that which describes our love of one another as a step toward the love of God.
Aquinas followed Aristotle very closely in most matters. He took over many of Aristotle's arguments as his own, following the same basic course of reasoning. The priority of the intellectual virtues over the moral becomes in the hands of Aquinas the priority of our relation to God over the practical governance of our lives. The highest good is the contemplation of God. God is the noblest object of contemplation and contemplation is the noblest activity of the soul. Our end is to become like God, but insofar as God is pure spirit, this end is attainable only after this life, when we are released from our bondage to the body.
I have represented medieval Christian ethics as an assimilation of Greek philosophy to Christian religious views. I want to elaborate on the role of reason in this unified philosophy. In the philosophy of Plato and Aristotle, human beings act with reason when they act for the good. Further, it was held that anyone knowing what the good is would not fail to act for the good. In principle, a person could always act rationally and thus attain, through his own efforts alone, a kind of moral perfection.
The notion that a human could reach moral perfection with God's aid was heretical to Christian thinkers (literally heretical; it was called the Pelagian heresy). God's grace is an absolutely necessary condition for salvation. But what role does God's grace play in the scheme of things? How much depends on the choice made by God and how much on the choices made by us. This is a version of the problem of predestination. God creates humans knowing fully well to what degree they will be blessed with grace, so what place is there for human freedom?
Augustine and Aquinas struggled with this difficulty, though we will not be able to examine their specific solutions. Both wanted to make freedom of the will compatible with God's acts of bestowing grace. Some Christian reformers, most notably John Calvin, took the position at the other extreme from Pelagianism: the choices of the individual are utterly irrelevant to salvation: all depends on the grace of God. In Hobbes we will find a philosopher who attempted to reconcile human liberty with the pre-determination of all human acts.