32 Abraham, indeed, cannot be defended, if, as Faustus says, he wished to get children by Hagar, because he had no faith in God, who promised that he should have children by Sara. But this is an entire mistake: this promise had not yet been made. Any one who reads the preceding chapters will find that Abraham had already got the promise of the land with a countless number of inhabitants, but that it had not yet been made known to him how the seed spoken of was to be produced, whether by generation from his own body, or from his choice in the adoption of a son, or, in the case of its being from his own body, whether it would be by Sara or another.
Whoever examines into this will find that Faustus has made either an imprudent mistake or an impudent misrepresentation. Abraham, then, when he saw that he had no children, though the promise was to his seed, thought first of adoption. This appears from his saying of his slave, when speaking to God, "This is mine heir;" as much as to say, As You have not given me a seed of my own, fulfill Your promise in this man. For the word seed may be applied to what has not come out of a man's own body, else the apostle could not call us the seed of Abraham: for we certainly are not his descendants in the flesh; but we are his seed in following his faith, by believing in Christ, whose flesh did spring from the flesh of Abraham.
Then Abraham was told by the Lord "This shall not be your heir; but he that comes out of your own bowels shall be your heir." The thought of adoption was thus removed; but it still remained uncertain whether the seed which was to come from himself would be by Sara or another. And this God was pleased to keep concealed, till a figure of the Old Testament had been supplied in the handmaid. We may thus easily understand how Abraham, seeing that his wife was barren, and that she desired to obtain from her husband and her handmaid the offspring which she herself could not produce, acted not in compliance with carnal appetite, but in obedience to conjugal authority, believing that Sara had the sanction of God for her wish; because God had already promised him an heir from his own body, but had not foretold who was to be the mother.
Thus, when Faustus shows his own infidelity in accusing Abraham of unbelief, his groundless accusation only proves the madness of the assailant. In other cases, Faustus' infidelity has prevented him from understanding; but here, in his love of slander, he has not even taken time to read.
Source: Reply to Faustus the Manichaean (New Advent)