1 After this fashion also did a presbyter, a disciple of the apostles, reason with respect to the two testaments, proving that both were truly from one and the same God. For [he maintained] that there was no other God besides Him who made and fashioned us, and that the discourse of those men has no foundation who affirm that this world of ours was made either by angels, or by any other power whatsoever, or by another God. For if a man be once moved away from the Creator of all things, and if he grant that this creation to which we belong was formed by any other or through any other [than the one God], he must of necessity fall into much inconsistency, and many contradictions of this sort; to which he will [be able to] furnish no explanations which can be regarded as either probable or true. And, for this reason, those who introduce other doctrines conceal from us the opinion which they themselves hold respecting God, because they are aware of the untenable and absurd nature of their doctrine, and are afraid lest, should they be vanquished, they should have some difficulty in making good their escape. But if any one believes in [only] one God, who also made all things by the Word, as Moses likewise says, “God said, Let there be light: and there was light;” and as we read in the Gospel, “All things were made by Him; and without Him was nothing made;” and the Apostle Paul [says] in like manner, “There is one Lord, one faith, one baptism, one God and Father, who is above all, and through all, and in us all” — this man will first of all “hold the head, from which the whole body is compacted and bound together, and, through means of every joint according to the measure of the ministration of each several part, makes increase of the body to the edification of itself in love.” And then shall every word also seem consistent to him, if he for his part diligently read the Scriptures in company with those who are presbyters in the Church, among whom is the apostolic doctrine, as I have pointed out.
2. For all the apostles taught that there were indeed two testaments among the two peoples; but that it was one and the same God who appointed both for the advantage of those men (for whose sakes the testaments were given) who were to believe in God, I have proved in the third book from the very teaching of the apostles; and that the first testament was not given without reason, or to no purpose, or in an accidental sort of manner; but that it subdued those to whom it was given to the service of God, for their benefit (for God needs no service from men), and exhibited a type of heavenly things, inasmuch as man was not yet able to see the things of God through means of immediate vision; and foreshadowed the images of those things which [now actually] exist in the Church, in order that our faith might be firmly established; and contained a prophecy of things to come, in order that man might learn that God has foreknowledge of all things.
Source: Adversus haereses (New Advent)