The Manichees are accustomed to find fault in the following way with the first book of the Old Testament, which is entitled, Genesis. About the words, “In the beginning God made heaven and earth,” they ask, “In what beginning?” They say, “If God made heaven and earth in some beginning of time, what was he doing before he made heaven and earth? And why did he suddenly decide to make what he had not previously made through eternal time?” We answer them that God made heaven and earth in the beginning, not in the beginning of time, but in Christ. For he was the Word with the Father, through whom and in whom all things were made. For, when the Jews asked him who he was, our Lord Jesus Christ answered, “The beginning; that is why I am speaking to you.”
But even if we believe that God made heaven and earth at the beginning of time, we should certainly realize that there was no time before the beginning of time. For God also made time, and thus there was no time before he made time. Hence, we cannot say that there was a time when God had not yet made anything. For how could there be a time that God had not made since he is the maker of all time? And if time began to be with heaven and earth, there cannot be found a time when God had not yet made heaven and earth.
When they say, “Why did he suddenly decide?” they speak as if some time passed during which God produced nothing. But a time could not pass that God had not already made, because he cannot be the producer of time unless he is before time. Surely the Manichees themselves read the Apostle Paul and praise and honor him, and they mislead many by interpreting his Letters wrongly. Let them tell us what the Apostle Paul meant when he said, “The knowledge of the truth which is in accord with the goodness of God for the hope of eternal life, which God who cannot lie, promised before eternal time.” For what could precede eternal time? Let them be forced to explain this. Then they will understand that they do not understand when they rashly want to find fault with what they ought to study with care.
Augustine of Hippo, On Genesis: Two Books on Genesis against the Manichees, 84:49–50.