Bede : The Lord visited Sarah as He had promised, and fulfilled what He had spoken to her; she conceived and bore a son in her old age at the time God had predicted to her. God is said to visit Sarah as one who is sick and already despaired of all offspring, so that what nature seemed to deny, the presence of divine grace bestows: which word aptly suits all the children of the promise, who are saved not by their own free will but by the choice of grace, not by the effort of their own labor but by the Lord visiting their hearts and fulfilling the gift of grace which He promised. — Commentary on Genesis (Hexaemeron) John Chrysostom : Do you wish to learn the symbolic meaning of Sarah's sterility? The church was to bring forth the multitude of believers. In order, therefore, that you may not find incredible how one who was childless, fruitless and barren could have given birth, she who by nature was barren went ahead, paving the way for chosen sterility, and Sarah became a type of the church. For just as she gave birth in her old age when she was barren, so too the church, though barren, has given birth for these, the final times. — DO NOT DESPAIR