By Abraham's listening to his wife, who had not believed God as he had, he was brought down to her level, the old level of natural cause and effect (Gen. 16). According to nature, Abraham's belief was folly. So it appeared to Sarah.
Accordingly, she took the common sense view of the matter, and said the thing must be brought about by natural means. However, the purposes of God are not wrought out in this way, nor does the man of faith live the life appointed for him in this manner.
The result of natural cause is natural effect; the bondwoman produced a son who by nature was not, and could never be, the son of promise. He lived and became the father of a race always at enmity with the seed of promise. This is God's way of pointing out that the just must live by faith.
It is no use attempting to bring God's purposes to pass by our own wills, in our own time and by natural means. The only result will be a crop of thorns and many bitter reflections. Still God will use even our crop of thorns to His own ends and our blessing. His government is one of grace.