Thomas Aquinas, Compendium of Theology

To restore man, who had been laid low by sin, to the heights of divine glory, the Word of the eternal Father, though containing all things within His immensity, willed to become small. This He did, not by putting aside His greatness, but by taking to Himself our littleness. No one can say that he is unable to grasp the teaching of heavenly wisdom; what the Word taught at great length, although clearly, throughout the various volumes of Sacred Scripture for those who have leisure to study, He has reduced to brief compass for the sake of those whose time is taken up with the cares of daily life. Man's salvation consists in knowing the truth, so that the human mind may not be confused by divers errors; in making for the right goal, so that man may not fall away from true happiness by pursuing wrong ends; and in carrying out the law of justice, so that he may not besmirch himself with a multitude of vices.

Knowledge of the truth necessary for man's salvation is comprised within a few brief articles of faith. The Apostle says in Romans 9:28: "A short word shall the Lord make upon the earth"; and later he adds: "This is the word of faith, which we preach" (Rom. 10:8). In a short prayer Christ clearly marked out man's right course; and in teaching us to say this prayer, He showed us the goal of our striving and our hope. In a single precept of charity He summed up that human justice which consists in observing the law: "Love therefore is the fulfilling of the law" (Rom. 13:10). Hence the Apostle, in I Corinthians 13:13, taught that the whole perfection of this present life consists in faith, hope, and charity, as in certain brief headings outlining our salvation: "Now there remain faith, hope, and charity." These are the three virtues, as St. Augustine says, by which God is worshiped.

Wherefore, my dearest son Reginald, receive from my hands this compendious treatise on Christian teaching to keep continually before your eyes. My whole endeavor in the present work is taken up with these three virtues. I shall treat first of faith, then of hope, and lastly of charity. This is the Apostle's arrangement which, for that matter, right reason imposes. Love cannot be rightly ordered unless the proper goal of our hope is established; nor can there be any hope if knowledge of the truth is lacking. Therefore the first thing necessary is faith, by which you may come to a knowledge of the truth. Secondly, hope is necessary, that your intention may be fixed on the right end. Thirdly, love is necessary, that your affections may be perfectly put in order.

PART ONE FAITH