Many men know not what is in them, or rather what is ready for them, until they are put upon what seems utterly above their strength ; indeed, upon what is really above their strength. The duties that God, in an ordinary way, requires at our hands are not proportioned to what strength we have in ourselves, but to what help and relief is laid up for us in Christ; and we are to address ourselves to the greatest performances with a settled persuasion that we have not ability for the least. This is the law of grace ; but yet, when any duty is required that is extraordinary, that is a secret not often discovered. In the yoke of Christ it is a trial, a temptation.
from Of Temptation; volume 6 of Works, page 94