"They cannot allege that they make this vow trusting entirely to the grace of God; for, seeing he declare this to be a special gift not given to all (Matthew 19:11), no man has a right to assume that the gift will be his."
-Institutes of the Christian Religion, IV.xii.17
And later,
"If any seem more decent and modest than others, they are not, however, chaste. The sin of unchastity urges, and lurks within. Thus it is that God, by fearful examples, punishes the audacity of men, when, unmindful of their infirmity, they, against nature, affect that which has been denied to them, and despising the remedies which the Lord has placed in their hands, are confident in their ability to overcome the disease of incontinence by contumacious obstinacy."
-Institutes, IV.xiii.21