Deuteronomy 6:4

For Moses, the faithful servant of God, when he said, “The Lord thy God is one Lord,” and thus proclaimed that there was only one God, did yet forthwith confess also our Lord when he said, “The Lord rained upon Sodom and Gomorrah fire and brimstone from the Lord.” And again, “And God said, Let Us make man after our image: and so God made man, after the image of God made He him.” And further, “In the image of God made He man.” And that [the Son of God] was to be made man, [Moses shows when] he says, “A prophet shall the Lord raise up unto you of your brethren, like unto me.”

There is then one God and Father, and not two or three; One who is; and there is no other besides Him, the only true [God]. For “the Lord thy God,” saith [the Scripture], “is one Lord.” And again, “Hath not one God created us? Have we not all one Father? And there is also one Son, God the Word. For “the only-begotten Son,” saith [the Scripture], “who is in the bosom of the Father.” And again, “One Lord Jesus Christ.” And in another place, “What is His name, or what His Son’s name, that we may know?” And there is also one Paraclete. For “there is also,” saith [the Scripture], “one Spirit,” since “we have been called in one hope of our calling.” And again, “We have drunk of one Spirit,” with what follows. …

2. Again, our Lord Jesus Christ confesses this same Being as His Father, where He says: “I confess to thee, O Father, Lord of heaven and earth.” What Father will those men have us to understand [by these words], those who are most perverse sophists of Pandora? Whether shall it be Bythus, whom they have fabled of themselves; or their Mother; or the Only-begotten? Or shall it be he whom the Marcionites or the others have invented as god (whom I indeed have amply demonstrated to be no god at all); or shall it be (what is really the case) the Maker of heaven and earth, whom also the prophets proclaimed, whom Christ, too, confesses as His Father, whom also the law announces, saying: “Hear, O Israel; The Lord thy God is one God?”

1. Thus then does the Lord plainly show that it was the true Lord and the one God who had been set forth by the law; for Him whom the law proclaimed as God, the same did Christ point out as the Father, whom also it behoves the disciples of Christ alone to serve. By means of the statements of the law, He put our adversary to utter confusion; and the law directs us to praise God the Creator ( Demiurgum ), and to serve Him alone. Since this is the case, we must not seek for another Father besides Him, or above Him, since there is one God who justifies the circumcision by faith, and the uncircumcision through faith. For if there were any other perfect Father above Him, He (Christ) would by no means have overthrown Satan by means of His words and commandments. For one ignorance cannot be done away with by means of another ignorance, any more than one defect by another defect. …

But do you want to hear from the Greeks expressly of one first principle? Timæus the Locrian, in the work on Nature, shall testify in the following words: “There is one first principle of all things unoriginated. For were it originated, it would be no longer the first principle; but the first principle would be that from which it originated.” For this true opinion was derived from what follows: “Hear,” it is said, “O Israel; the Lord thy God is one, and Him only shalt thou serve.”

But not yet about the good to be got from martyrdom must we learn, without our having first heard about the duty of suffering it; nor must we learn the usefulness of it, before we have heard about the necessity for it. The (question of the) divine warrant goes first, whether God has willed and also commanded ought of the kind, so that they who assert that it is not good are not plied with arguments for thinking it profitable save when they have been subdued. It is proper that heretics be driven to duty, not enticed. Obstinacy must be conquered, not coaxed. And, certainly, that will be pronounced beforehand quite good enough, which will be shown to have been instituted and also enjoined by God. …

One God , whose mighty voice to Moses spake

And now, indeed, concerning the Father, and the Son, and the Holy Spirit, let it be sufficient to have briefly said thus much, and to have laid down these points concisely, without carrying them out in a lengthened argument. For they could be presented more diffusely and continued in a more expanded disputation, since the whole of the Old and New Testaments might be adduced in testimony that thus the true faith stands. But because heretics, ever struggling against the truth, are accustomed to prolong the controversy of pure tradition and Catholic faith, being offended against Christ; because He is, moreover, asserted to be God by the Scriptures also, and this is believed to be so by us; we must rightly, that every heretical calumny may be removed from our faith, contend, concerning the fact that Christ is God also, in such a way as that it may not militate against the truth of Scripture; …

VI . Let not a bishop be given to filthy lucre, especially before the Gentiles, rather suffering than offering injuries; not covetous, nor rapacious; no purloiner; no admirer of the rich, nor hater of the poor; no evil-speaker, nor false witness; not given to anger; no brawler; not entangled with the affairs of this life; not a surety for any one, nor an accuser in suits about money; not ambitious; not double-minded, nor double-tongued; not ready to hearken to calumny or evil-speaking; not a dissembler; not addicted to the heathen festivals; not given to vain deceits; not eager after worldly things, nor a lover of money. For all these things are opposite to God, and pleasing to demons. Let the bishop earnestly give all these precepts in charge to the laity also, persuading them to imitate his conduct. …

XX . Now the law is the decalogue, which the Lord promulgated to them with an audible voice, before the people made that calf which represented the Egyptian Apis. And the law is righteous, and therefore is it called the law, because judgments are thence made according to the law of nature, which the followers of Simon abuse, supposing they shall not be judged thereby, and so shall escape punishment. This law is good, holy, and such as lays no compulsion in things positive. For He says: “If thou wilt make me an altar, thou shalt make it of earth.” It does not say, “Make one,” but, “If thou wilt make.” It does not impose a necessity, but gives leave to their own free liberty. For God does not stand in need of sacrifices, being by nature above all want. …

XXIII . For He did not take away the law of nature, but confirmed it. For He that said in the law, “The Lord thy God is one Lord;” the same says in the Gospel, “That they might know Thee, the only true God.” And He that said, “Thou shalt love thy neighbour as thyself,” says in the Gospel, renewing the same precept, “A new commandment I give unto you, that ye love one another.” He who then forbade murder, does now forbid causeless anger. He that forbade adultery, does now forbid all unlawful lust. He that forbade stealing, now pronounces him most happy who supplies those that are in want out of his own labours. He that forbade hatred, now pronounces him blessed that loves his enemies. He that forbade revenge, now commands long-suffering; not as if just revenge were an unrighteous thing, but because long-suffering is more excellent. …

12. But whereas, in the same Trinity, some things severally are specially predicated, these are in no way said in reference to themselves in themselves, but either in mutual reference, or in respect to the creature; and, therefore, it is manifest that such things are spoken relatively, not in the way of substance. For the Trinity is called one God, great, good, eternal, omnipotent; and the same God Himself may be called His own deity, His own magnitude, His own goodness, His own eternity, His own omnipotence: but the Trinity cannot in the same way be called the Father, except perhaps metaphorically, in respect to the creature, on account of the adoption of sons. …