John Calvin Commentary Isaiah 58:5

"Is such the fast that I have chosen? the day for a man to afflict his soul? Is it to bow down his head as a rush, and to spread sackcloth and ashes under him? wilt thou call this a fast, and an acceptable day to Jehovah?" — Isaiah 58:5 (ASV)

Is it such a fast as I have chosen? He confirms the preceding statement and shows that fasting is neither desired nor approved by God in itself, but only insofar as it is directed to its true end. He did not wish for it to be altogether abolished, but rather its improper use—that is, because they believed the worship of God consisted in it and, by neglecting or even despising true godliness, thought that bodily exercise was enough, just as hypocrites always put forward external ceremonies as if they were means of appeasing God.

Again, because people, in their rashness, define what the worship of God is, He explicitly refers us to His own will, so that we do not suppose that He approves of everything which our own judgment pronounces to be right. Although people are well pleased with themselves, swell with astonishing haughtiness, and indulge in insolent boasting, the Lord rejects and abhors them, because He claims for Himself alone the right to choose. Now, to choose a thing has the same meaning as to take pleasure in it.

And hanging his head like a bulrush. He says that He is not delighted if someone passes a day in hunger and then walks with a sad and downcast look. The Prophet employs an appropriate metaphor, because the bulrush, though straight, is easily bent. So hypocrites bend themselves and bow down their heads, as if under the effects of debilitating hunger, or to display some empty show of humility. The Prophet therefore intended to censure such superstitious affectations, in which hypocrites imagine there is some holiness.

And spread sackcloth and ashes. These things were also added to fasting, especially when people made solemn professions of repentance, for they clothed themselves with sackcloth and threw ashes on their heads (Joel 1:13). Now, such a practice was holy and approved by God, and we see that the prophets, while exhorting the people to repentance, called for sackcloth and ashes. But, as we have said that fasting is not condemned here in itself, so Isaiah does not condemn these outward ceremonies but reproves hypocrites for separating them from the genuine repentance they should signify.

If it is asked, "Are sackcloth and ashes suitable for our time?" I reply, they are indifferent matters, which may be used for edification. But in the light of the Gospel, which has brought liberty to us, we have no need of such symbolic practices.

At the same time, we should consider the difference between Eastern nations, which use a great abundance and variety of ceremonies, and Western nations, whose customs are far simpler. If we wished to imitate the former, it would be nothing more than to act like apes or stage actors. Yet there is nothing to prevent those who intend to confess their guilt from wearing soiled and humble garments, in the manner of supplicants.

A day acceptable to Jehovah. Thus, it is evident that fasting was added to solemn prayer when a holy assembly was held, for fasting, as we have already said, is an appendage to prayer, as we see that it was added to prayer by Christ Himself (Matthew 17:21). Therefore, it is not appointed for its own sake but is directed toward a different purpose.