In this prologue St. Thomas shows the place assigned to this treatise in his Theological Summa, according to the division made by him at the beginning of this work, in which he had said: "Because the chief aim of sacred doctrine is to teach the knowledge of God, not only as He is in Himself, but also as He is in the beginning of things and their last end, and especially of rational creatures... we shall treat:
(1) "Of God (one in nature and triune in persons, and inasmuch as He is the principle of creatures); (2) of the rational advance of creatures toward God (or of God as He is the end of the rational creature); (3) of Christ, who as man is our way to God."
In the present treatise he says: "Because our Savior the Lord Jesus Christ in order to save His people from their sins, as the angel announced, showed unto us in His own person the way of truth, whereby we may attain to the bliss of eternal life by rising again, it is necessary... that, after consideration of the last end of human life, and the virtues and vices, there should follow a consideration of the Savior of all and of the benefits bestowed by Him on the human race."
Some theologians prefer another division to that made by St Thomas, in which the distinction between dogmatic theology and moral theology is more in evidence, so that moral theology is not placed between the treatises on the One God and the Word incarnate. Furthermore, they remark that the treatise on the Word incarnate because of its dignity justly comes immediately after the treatise on the one and triune God.
To this the Thomists reply that, according to St. Thomas, dogmatic theology and moral theology are not two distinct sciences, but two parts of the same science, similar to the science of God of which it is a participation. The unity of this science results from the unity of its formal object both quod and quo. Its formal object quod, or the subject of this science, is God Himself considered in Himself, or as He is the principle and end of creatures. The formal object quo is virtual revelation by the light of which are deduced both in dogmatic theology and moral theology the conclusions that are virtually contained in the revealed principles. Therefore dogmatic and moral theology are not two sciences, but two parts of the same science.
They also remark that, although this treatise on the Savior, because of its dignity, precedes the moral part of theology, nevertheless, in the orderly arrangement of knowledge, it is justly placed after the other parts of theology, and this especially for three reasons: (1) because the simpler things come before the composite. In the preceding parts of the Summa, however, what pertains to God and to man are discussed separately, whereas the present treatise is concerned with Him who is both God and man. (2) The work of redemption presupposes also that man lived for a long period of time under the law of the Old Testament, as well as it presupposes acts of faith and other virtues necessary in the various states of life. Hence St. Thomas appropriately places this treatise on the Savior at the end of his Summa. (3) Moreover, it must be noticed that what is necessary precedes what is contingent. But in the two preceding parts of the Theological Summa, what forms the subject of special discussion is the nature of God, and the nature of both angels and man with reference to God; whereas the Third Part of the Summa considers the great contingent fact which did not have to be realized, namely, that the Word was made flesh. This fact, although it is the greatest of all historical facts in the universe, is a contingent fact; for it is not something absolutely necessary, such as the divine nature for God and also the human nature for man. For this reason, certain philosophers, even certain mystics, desired to reach union with God, not by way of Christ the universal mediator, although He had said: "I am the way and the truth and the life." These persons did not grasp the practical import of the statement that Christ, or the Word of God incarnate, is the exemplar and source of all virtues, without whom nobody can acquire salvation and sanctity.
This deviation from the common method of approach to God is in itself manifestly in opposition to the great truth, namely, that these persons somehow overlooked the fact of the Incarnation, inasmuch as it is not an absolutely necessary fact, and they failed to see that precisely because it is contingent, it becomes, in some aspect, a fact of the greatest importance, inasmuch as it is a transcendent manifestation of God's most free and absolutely gratuitous love for the human race. St. John testifies that: "God so loved the world as to give His only-begotten Son." He also says: "He hath first loved us, and sent His Son to be a propitiation for our sins." In fact, these texts express the fundamental truth of Christianity, which is that God, by a most free act of His love, sent His divine Son to us. Hence the entire third part of the Theological Summa of St. Thomas is a detailed narrative of God's gratuitous love for us confirmed by the text: "God so loved the world, as to give His only-begotten Son." It is truly a complete description of this gratuitous love as being the motive of God's mercy, and of the efficacy of this love. It is a canticle of God's gratuitous love for the human race. Thus the contingency of this most prominent fact in the history of the human race does not lessen its importance, but it manifests, on the contrary, the supreme gratuitousness of God's most free love for us.
Indeed, this manifestation of love is of such excellence that, in these days, even the more obnoxious enemies of the Church, such as several idealists, disciples of Hegel and Renan, who deny the existence of a true God really and essentially distinct from the world, say that Christ was the noblest of all men and that nobody was a better type of the evolution of the human race. So wrote Renan. In fact, several communists in these days say the same, and they furthermore remark that this evolution of the human race predicted by Christ can be realized only by communism. Thus, presenting Christ in an entirely false light, whether they wish it or not, they confess that the greatest event in the history of the human race was the coming of Christ. But before this statement about Christ can be understood, one must have a correct notion of both God and man. Hence this treatise on the Incarnation is logically placed in the third part of the Theological Summa.
From the prologue we see that St. Thomas divides the third part of his Summa by considering: (1) the Savior Himself; (2) the sacraments by which we attain to our salvation; (3) the end of immortal life to which we attain by the resurrection.
Thus it is evident that the third part of the Summa is a treatise on the Savior, and the benefits He bestowed on us by instituting the sacraments and enabling us to get to heaven, which is our last end.
The treatise on the Savior is divided into two parts.
Part I discusses the mystery itself of the Incarnation (q. 1-26).
Part II discusses the actions and sufferings of our Savior or the mysteries of the life of Christ (q. 27-59).
The first part is often called, in our days, Christology, and the second part is known as soteriology. The mystery of the Incarnation is the principal topic of discussion in the first part, and in the second part St. Thomas considers the mystery of Redemption, in which he discusses especially the passion of Christ (q. 46-52).
The first part of the mystery of the Incarnation contains three sections:
1) The fitness of the Incarnation, in which it is discussed as a historical fact (q. 1).
2) The mode of union of the Word incarnate is considered (q. 2-15). The union itself (q. 2), the union in its relation to the person assuming (q. 3), and then on the part of the nature assumed and its perfections, the grace, knowledge, and powers of Christ are discussed (q. 4-15).
3) The consequences of the union with reference to what belongs to Christ are here discussed: (1) in themselves (q. 16-19); (2) in their relation to the Father, in which the predestination of Christ is considered; (3) with reference to us, in which our adoration of Christ and His office of Mediator are discussed (q. 25-26).
The second part is concerned with the mysteries of the life of Christ, and is divided into four sections: (1) the coming of Christ into the world, which includes Mariology; (2) His life on earth in its gradual development; (3) the end of His life, or His passion and death; (4) His exaltation, or His resurrection and ascension.
This second part which is entitled, The Mystery of Redemption, will be a brief treatise on the Passion, as it is the cause of our salvation, the vicarious satisfaction of Christ, its infinite value, Christ's victory, and also Christ as king, judge, and head of the blessed. Finally there will be a compendium on Mariology.
It must be noticed that among the commentators of the Summa John of St. Thomas discusses the satisfaction of Christ at the beginning of His commentary, by considering the fittingness of the Incarnation, inasmuch as the Son of God came down from heaven for our salvation, namely, to redeem the human race. This arrangement is, indeed, appropriate for a complete understanding of the thesis on the motive of the Incarnation. However, in the doctrinal order, so far as operation follows being, St. Thomas is justified in discussing the Incarnation before the Redemption, or before the theandric act of the love of Christ suffering for us. Probably the reason why John of St. Thomas discussed at length the satisfaction of Christ at the beginning of his commentary, is that it ends with the twenty-fourth question in the Summa of St. Thomas.
Billuart, however, developed his thesis on the satisfaction of Christ in connection with the merit of Christ, which is question nineteen in the Summa of St. Thomas, at the same time discussing the infinite value of the merits of Christ.
Following the arrangement of questions as given by St. Thomas, we shall consider: (1) the mystery of the Incarnation; (2) the mystery of Redemption. This is the method commonly adopted by theologians.