All the stability of a promise depends upon the qualifications of the promiser to the ends and purposes of the promise. If a man make me a promise to do such and such things for me, and I question whether ever it will be so or not, it must be from a doubt of the want of one of these things in him that makes the promise;—either
(1.) of truth; or
(2.) of ability to make good his word, because of the difficulty of the thing itself; or
(3.) of sincerity to intend me really what he speaks of; or
(4.) of constant memory to take the opportunity of doing the thing intended ; or
(5.) of stableness to be still of the same mind.
Now, if there be no want of any of these in him whose promises we speak of, there is then certainly no ground of our staggering, but only from our own unbelief.
from a sermon entitled The Steadfastness of the Promises, and Sinfulness of Staggering; volume 8 of Works, page 221