The command of Christ
Now whatever reasons there were to quicken those of old in the zealous and constant discharge of this duty, they are of equal force still to quicken us. But above all these we have a peculiar reason for being in fastings often, namely, the command of him by whose name we are called. He does not indeed in this place expressly enjoin either fasting, giving of alms, or prayer. But his directions how to fast, to give alms, and to pray are of the same force with such injunctions. For the commanding us to do anything thus is an unquestionable command to do that thing, seeing it is impossible to perform it thus if it be not performed at all. Consequently the saying, give alms, pray, fast in such a manner, is a clear command to perform all those duties, as well as to perform them in that manner which shall in no wise lose its reward.
1771 text
12. Now whatsoever reasons there were to quicken those of old in the zealous and constant discharge of this duty, they are of equal force still to quicken us. But above all these we have a peculiar reason for being in fastings often, namely, the command of him by whose name we are called. He does not indeed in this place expresly enjoin, either fasting, giving of alms, or prayer. But his directions how to fast, to give alms, and to pray, are of the same force with such injunctions. For the commanding us, to do any thing thus, is an unquestionable command, to do that thing; seeing it is impossible to perform it thus, if it be not performed at all. Consequently, the saying, give alms, pray, fast in such a manner, is a clear command to perform all those duties, as well as to perform them in that manner, which shall in no wise lose its reward.