Below is an extreme abridgment of Frederick Bauerschmidt's Holy Teaching: Introducing the Summa Theologiae of St. Thomas Aquinas, a Kreeft-like book of heavily annotated selections from the Summa, in his own flowing translation. A sentence has been selected from each of its 43 chapters. Interested readers are referred to Bauerschmidt's original 320-page work.
In order that the salvation of human beings might be brought about more fittingly and more surely, it was necessary that they should be taught divine truths by divine revelation. The existence of God can be proved in five ways. God is his own existence, and not merely his own essence. And sometimes perceptible things, or even voices, are divinely formed to express some divine meaning, as in the baptism [of Christ], the Holy Spirit was seen in the shape of a dove, and the voice of the Father was heard: "This is my beloved Son" (Matthew 3:17).
Besides the procession of the Word there is posited another procession in God, which is the procession of love. The word "person" signifies the essence directly and the relation indirectly, since the essence is the same as they hypostasis. It is impossible to arrive at knowledge of the Trinity by natural reason. The Son proceeds by the way of the intellect as Word, and the Holy Spirit by way of the will as Love.
The human intellect will have its perfection through union with God as with that object in which alone perfect human happiness consists. The perfect condition of the body is required for happiness that is in every way perfect, and this is the case both as a condition for and as a result of happiness. Human beings are capable of seeing God, and perfect human happiness consists in this vision.
In order that human beings may know without any doubt what they should do and what they should avoid, it was necessary for them to be directed in their proper acts by a law given by God, for it is certain that such a law cannot be in error. In the state of corrupted nature one needs grace for two reasons: in order to be healed and, beyond this, in order to carry out works of supernatural virtue, which are meritorious.
Since grace has been revealed, both the great ones and the lowly are required to have explicit faith in the mysteries of Christ, particularly those that the whole church celebrates and publicly proclaims, such as the articles that refer to the incarnation. The proper and principal object of hope is eternal happiness. With regard to filial fear, just as it grows as charity grows, so too is it perfected when charity is made perfect. Charity is the friendship of human beings toward God.
Unless Christ was God, he would not have brought a remedy; and unless he was human, he would not have set an example. Since the Word has united a human nature to himself—which does not, however, belong to his divine nature—it follows that the union took place in the person of the Word, and not in the nature. Christ is fittingly called the head of the church. It must be said that in Christ there was acquired knowledge, which is truly knowledge in a human fashion. The word "human" may be truly and properly predicated of the word "God," on the ground that it stands for the person of the Son of God. Because in Christ there are two natures and one hypostasis, it follows that things belonging to the nature in Christ must be two, and that those belonging to the hypostasis in Christ must be only one.
It was fitting that Christ should give people confidence in approaching him by associating familiarly with them. If Christ had committed his teaching to writing, people would have thought his teaching as no more profound than what appears on the surface of the writing. Through his suffering human beings know how much God loves them, and are thereby stirred to love him in return, which is the perfection of human salvation. Since Christ's soul did not fend off the injury inflicted on his body, but rather willed his bodily nature to succumb to such injury, he is said to have "laid down his life," or to have died voluntarily. By suffering out of love and obedience, Christ gave more to God than was required to compensate for the offense of the whole human race. In dying, he endured evil things that he might deliver us from evil, so he was glorified in rising again in order to advance us toward good things. And there must be a final judgment at the last day, in which everything concerning every human being in every respect shall be fully and openly judged.
Through the institution of the sacraments human beings, in a way consistent with their nature, are instructed through perceptible things. The sacraments of the New Law cause grace, for they are instituted by God to be employed for the purpose of conferring grace. By means of the sacraments, the faithful receive a certain spiritual seal. It is fitting that there should be seven sacraments; for spiritual life has a certain resemblance to the life of the body. The sacrament of baptism does not occur in the water itself, but in applying the water to a human being, that is, in the washing. It became necessary to baptize children, so that, as in birth they incurred damnation through Adam, in a second birth they might obtain salvation through Christ.
The presence of Christ's true body and blood in the sacrament of the Eucharist cannot be detected by the senses but only by faith, which rests on divine authority. It sometimes happens that someone is hindered from receiving the effect of this sacrament, and such reception is an imperfect one. By this sacrament we are made partakers of the fruit of our Lord's suffering.