After the death of Abraham, God blessed Isaac his son. . .. These are the generations of Ishmael
[Moses] says that God blessed Isaac after the death of his father. But he soon breaks off, and in few words he completes what there is still to say about Ishmael. But he does not speak of the blessing [of Isaac] until the proper time for this has come. For it was delayed 20 years, during which Isaac lived with his wife Rebecca without offspring. . .. Reason dreams that God has to be worshiped and appeased with physical sacrifices or other exercises devised by men. But the examples of the fathers show that the foremost and best worship is to wait for God. And this is the real benefit and the most appropriate exercise of faith. For faith first carries us away into things that are invisible when it points out that things that are not apparent to the eye must be accepted. This we can somehow bear and put up with. The heart, however, is not only led into what is invisible; but it is also kept in suspense and is put off for a long time, just as Abraham ... waited for 25 years before a son was born to him, and Isaac is without offspring for 20 years. But the third and by far the most serious thing is experienced when delay and postponement are followed by a disposition to the opposite effect. It is then that he who is able to endure and wait, to hope for the things that are being delayed, and to be pleased with what is contrary, will eventually learn from experience that God is truthful and keeps His promises.