(60) Why does he say, But I will set up my covenant with Isaac, whom Sarah shall bring forth about this time in the succeeding year? (#Ge 17:21). As in men’s wills some persons are set down as heirs, and some are entered as worthy of gifts which they are to receive from the heirs, so also in the divine testament that man is set down as the heir who is by nature a worthy disciple of God being adorned with all perfect virtues; but he who is introduced by learning, and is made subject to the law of wisdom, and partakes in encyclical instruction, is not at all an heir, but only a receiver of gifts gratuitously given. But it is said with great wisdom and propriety that his mother shall bring forth Isaac in the succeeding year, since this birth unto life does not belong to the present time, but to another great and holy time; and that which is divine rejoices in excessive abundance, and is by no means like the nations of this world.
(61) Why does he say, Abraham was ninety and nine years old when he was circumcised, and Ishmael his son was thirteen years old? (#Ge 17:24). The number of ninety and nine years is arranged here as approximating to the number a hundred. And it is in accordance with this number that it is arranged that the seed of the perfect man becomes the beginning of generation, which appears more evidently in the number a hundred; but the number thirteen is composed of the first square numbers of four and nine, the odd and even numbers; so that the even number has for its sides a twofold material form; and the odd number has an operative form, from all which a triple number is made, which is the greatest and most perfect of the festival victims which the examinations of the sacred scriptures contain. This is one reason. A second also it may be allowed to us to mention, that the age namely of thirteen years is very near to and a partaker with the fourteenth year, in which the motions of seed towards generation begin to have life. In order, therefore, that no foreign seed should be sown, he arranged that the first generations should be kept pure, figuring the instrument of generating under the figure of generation. In the third place, he teaches that he who is about to go through the operations of matrimony ought by all means first of all to cut away concupiscence, reproving all lascivious and effeminate persons as those who bring together superfluous mixtures which were not for the sake of the generation of children but to gratify incontinent desires.
(62) Why did Abraham also circumcise strangers? (#Ge 17:27). The wise man is as useful as the humane man, who saves and invites to himself not only his relations and neighbours, but also strangers and men of another family, giving them a share of his own habit of patient and religious continence; for these are the foundations of constancy, which is the object of all virtue, and the point at which it rests.
