Against Heresies: Book IV
Chapter 32: Confirmation from an Apostle-Trained Presbyter that One God Authored Both Testaments
In a similar manner, a presbyter—one of the apostles' disciples—reasoned about the two testaments, showing that both came from the same God. He asserted that no other God exists besides Him who created and designed us, rejecting the claims of those who say this world was made by angels, any other power, or another God. If a person strays from believing in the Creator of all things, and admits that this creation, to which we belong, was formed by anyone else, they will inevitably encounter many contradictions and inconsistencies.
They won't be able to offer explanations that are even remotely plausible or true. This is why those who introduce other doctrines hide their opinions about God, knowing how weak and absurd their beliefs are, and they fear being defeated and struggling to defend themselves.
However, if someone believes in one God, who made all things by the Word—as Moses 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 nothing was made;" and the Apostle Paul similarly states, "There is one Lord, one faith, one baptism, one God and Father, who is above all, through all, and in us all"—this person will first of all "hold the head, from which the whole body is fitted and held together, and, through every joint according to the provision of each part, causes the growth of the body to build itself up in love." Then every word will seem consistent to him, especially if he diligently reads the Scriptures with presbyters in the Church, among whom is the apostolic doctrine, as I have indicated.
The apostles all taught that there were indeed two testaments for the two peoples, but it was one and the same God who appointed both for the benefit of those who were meant to believe in God. I have shown this in the third book from the teachings of the apostles. The first testament wasn't given without reason, nor without purpose, nor accidentally. It was meant to guide those to whom it was given to serve God for their own benefit, since God doesn't need service from men. It presented a model of heavenly things, as people were not yet able to directly see the things of God. It foreshadowed what now exists in the Church to strengthen our faith, and it contained a prophecy of things to come, so that people might understand that God knows everything in advance.