The truth is, the question is not about the nature of the thing, but about the power of God: Place the doubt aright, and it is this: Is God able to accomplish what he hath spoken? —can he heal my backslidings? can he pardon my sins? can he save my soul? Now, that there may be no occasion or color of staggering upon this point, you see God reveals himself as an all-sufficient God, as one that is able to go through with all his engagements. If you will stagger, you may so do. This is certain, you have no cause to do so from hence,—there is not any promise that ever God entered into but he is able to perform it.
from a sermon entitled The Steadfastness of the Promises, and Sinfulness of Staggering; volume 8 of Works, page 225