After Abraham was commanded to send Hagar and Ishmael away:
“When the water in the skin was gone, she cast the child under one of the bushes. Then she went, and sat down over against him a good way off, about the distance of a bowshot; for she said: Let me not look upon the death of the child. And as she sat over against him, she lifted up her voice and wept.” Gen 21:15-16
If someone wanted to rant against Abraham at this point, he could make him the murderer of his son and wife. The responsibility for this outcome lies in him who, as it seems, is cruelly casting out his own flesh. And Moses himself employs rather horrifying words; he states that Ishmael, now almost dead from thirst, was laid under a bush by his mother, and that her motherly affection constrained her to withdraw from him, lest she see her son breathe his last.
Who would believe this if Moses had not recorded it? Clearly one misfortune follows on the heels of the other. The first-born son was cast out from his father’s house and was deprived of all of his father’s wealth, ln addition to this, in the desert he is now in peril of his life because of hunger and thirst.
Moreover, it is a tragic misfortune that the mother lays the lad in the grass under a bush; for she is so heartbroken through grief that she cannot bear the sight of her dying son. Someone will say: “Abraham should have thought of these misfortunes beforehand and should not have acted so hastily, especially in a matter altogether contrary to natural love or affection.”
I have stated above that we should take very careful note of this example, in order that we may not argue when God commands something but may obey Him without delay. It will certainly not be easy for us to imitate this obedience which Abraham showed here, but what the monks are doing—wearing cowls and abstaining from meat and the handling of money—this will be very easy for us to imitate.
Therefore Abraham was no ordinary Christian or confessor, if I may express myself in this way. No, he was a martyr of martyrs. For who is there who does not know how intense a father’s affection toward his children and wife is? It is easier for a parent to suffer death than to forsake his own or to permit great harm to be done to them. But everything must yield to a command of God; and if you want to be a Christian, this is not a matter of wearing a black or gray garment. No, everything must be risked, not only wife and children but your own life.
For Christ teaches clearly (Matt. 10:37) that “he who loves father and mother more than Me is not worthy of Me.” This means: “When I come with My Word and command, then you must forget everything you have and possess in this whole world.”
Luther’s works, vol. 4: Lectures on Genesis: Chapters 21-25 p 40-41