John Chrysostom (attributed)

Isaac is interpreted ‘laughter,’ but the laughter of the saints is not the foolish convulsion of the lips, but the rational joy of the heart, which was the mystery of Christ. For as he was granted to his parents in their extreme age to their great joy, that it might be known that he was not the child of nature, but of grace, thus Christ also in this last time came of a Jewish mother to be the joy of the whole earth; the one of a virgin, the other of a woman past the age, both contrary to the expectation of nature.