We have already published treatises on the One God, the Triune God, the Creator, and the Holy Eucharist. These have been presented in the form of a commentary on the teaching of St. Thomas in his Theological Summa. It is the purpose of the present treatise on Christ the Savior to explain, in accordance with the more common interpretation of the Thomists, the teaching of St. Thomas on the motive of the Incarnation, the hypostatic union, and its effects. We have discussed at length the more difficult problems, such as the reconciliation of freedom with absolute impeccability in Christ, the intrinsically infinite value of His merits and satisfaction, His predestination with reference to ours, inasmuch as He is the first of the predestined, and the reconciliation, during the Passion, of the presence of extreme sorrow with supreme happiness experienced by our Lord in the summit of His soul. In all these problems our wish has been to manifest the unity of Christ inasmuch as He is one personal Being, although He has two really distinct and infinitely different natures. Hence the Person of Christ constitutes the one and only principle of all His theandric operations. In all these questions St. Thomas, according to his custom, wonderfully preserved the principle of economy[1] by reducing all things to the same principles and in the ultimate analysis to the one and only fundamental principle. Similarly, with reference to the Passion everything is reduced to the principle of the plenitude of grace. This plenitude, on the one hand, was the cause in the summit of our Lord's soul of the beatific vision and, on the other hand, it was the cause of His most ardent love as priest and victim, so that He willed to be overwhelmed with grief, and die on the cross a most perfect holocaust. At the end of this treatise we have given merely a compendium on Mariology, since a more complete commentary on this subject has recently been published by us in the French language. May the reading of these pages be a source of knowledge as well as of spiritual benefit to all students of theology.