6 Prayer is a strong piece of armour and a great security. You heard yesterday how the three children, fettered as they were, destroyed the power of the fire; how they trampled down the blaze; how they overcame the furnace, and conquered the operation of the element. Hear today again how the noble and great Isaac overcame the nature itself of bodies through prayer. They destroyed the power of fire, this man today loosed the bonds of incapacitated nature. And learn how he effected this.
“Isaac,” it says, “prayed concerning his wife, because she was barren.” This has today been read to you; yesterday the sermon was about prayer; and today again there is a demonstration of the power of prayer. See how the grace of the Spirit has ordered that what has been read today harmonises with what was said yesterday. “Isaac,” it says, “prayed concerning Rebecca his wife, because she was barren.” This first is worth inquiring into, for what cause she was barren. She was of a life admirable and replete with much chastity— both herself and her husband.
We cannot lay hold of the life of those just ones, and say that the barrenness was the work of sin. And not only was she herself barren, but also his mother Sarah, who had borne him; not only was his mother barren and his wife, but also his daughter-in-law, the wife of Jacob, Rachel. What is the meaning of this band of barren ones? All were righteous, all living in virtue, all were witnessed to by God. For it was of them that He said, “I am the God of Abraham, and the God of Isaac, and the God of Jacob.”
Of the same persons Paul also thus speaks. “For which cause God is not ashamed to call himself their God.” Many are the commendations of them in the New, many the praises of them in the Old Testament. On all sides they were bright and illustrious, and yet they all had barren wives, and continued in childlessness until an advanced period. When therefore you see man and wife living with virtue; when you see them beloved of God, caring for piety, and yet suffering the malady of childlessness; do not suppose that the childlessness is at all a retribution for sins.
For many are God's reasons for the dispensation, and to us inexplicable; and for all we must be heartily thankful, and think those only wretched who live in wickedness; not those who do not possess children. Often God does it expediently, though we know not the cause of events. On this account in every case it is our duty to admire His wisdom, and to glorify His unspeakable love of man.
Source: Homily Against Publishing the Errors of the Brethren (New Advent)