1555 · John Calvin, Commentary on a Harmony of the Evangelists Matthew, Mark, and Luke, part 2 Matt.17.21
This kind goeth not out, By this expression Christ reproved the negligence of certain persons, in order to inform them that it was not an ordinary faith which was required; for otherwise they might have replied that they were not altogether destitute of faith. The meaning therefore is, that it is not every kind of faith that will suffice, when we have to enter into a serious conflict with Satan, but that vigorous efforts are indispensably necessary. For the weakness of faith he prescribes prayer as a remedy, to which he adds fasting by way of an auxiliary. “You are effeminate exorcists,” said he, “and seem as if you were engaged in a mock-battle got up for amusement; but you have to deal with a powerful adversary, who will not yield till the battle has been fought out. Your faith must therefore be excited by prayer, and as you are slow and languid in prayer, you must resort to fasting as an assistance.”