We know not the power and strength that God puts forth in our behalf, nor what is the sufficiency of his grace, until, comparing the temptation with our own weakness, it appears unto us. The efficacy of an antidote is found when poison has been taken; and the preciousness medicines is made known by diseases. We shall never know what strength there is in grace if we know not what strength there is in temptation. We must be tried, that we may be made sensible of being preserved. And many other good and gracious ends he has, which he accomplishes toward his saints by his trials and temptations, not now to be insisted on.
– John Owen, from The Nature of Temptation