It is true, never did any wait upon God for the accomplishment and fulfilling of a promise, but he found many difficulties fall out between the word and the thing. So was it with Abraham in the business of a son : and so with David in the matter of a kingdom. God will have his promised mercies to fall as the dews upon the parched, gasping earth, or “as the shadow of a great rock in a weary land,” Isa. 32:2, —very welcome unto the traveler who has had the sun beat upon his head in his travel all the day.
from a sermon entitled The Steadfastness of the Promises, and Sinfulness of Staggering; volume 8 of Works, page 229