Luther continues the discussion of infertility in the marriage of Isaac and Rebekah, and highlights the challenge of persistent prayer.

In the same way Rebecca took refuge in earnest and persistent prayer and sighed anxiously night and day. Isaac, too, prayed for her and placed before God nothing else than that one trouble, namely, his wife’s barrenness. We should learn from this that all our troubles, even those that are physical, should be placed before God, but above all the spiritual needs. Isaac prayed in this way: “If it means the hallowing of Thy name, and if it tends to preserve Thy kingdom, give Rebecca offspring.” Where a promise is lacking, as Rebecca lacked it, prayer should supply this and should come to the rescue. But it is a difficult thing and requires great exertion. It is far more difficult than the preaching of the Word or other duties in the church. When we teach, we experience more than we do; for God speaks through us, and it is a work of God. But to pray is a most difficult work. Therefore it is also very rare.

-Luther, AE 4