(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.