In fact, that is precisely what the people have called their fast! They have offered public humility, bowed their heads low like marsh reeds, lay on the ground in scratchy sackcloth and filthy ashes. Surely, this is what our God desires; we have always fasted like this. We know that God asks this of us! But they are wrong, just as Micah and Hosea and an earlier Isaiah have said. This fast is not at all what God has in mind.
"Is not this the fast I choose: to loose the bonds of injustice, to undo the thongs of the yoke, to let the oppressed go free, and to break every yoke?" (58:6) Attention to justice, especially freedom for the oppressed, is God's definition of the fast these people ought to be doing. "Is it not to share your bread with the hungry, and bring the homeless poor into your house; when you see the naked to cover them, and not to hide yourself from your own kin?" (58:6) Public practices of false humility, great displays of bowing and scraping and the wearing of foul clothing, have nothing to do with the fast God has in mind. Perform deeds of justice, and you will be fasting as I would have you fast, proclaims this God.
"Then your light will break forth like the dawn, and your healing will spring up quickly; your vindicator will go before you, and the glory of YHWH shall be your rearguard" (58:8). I do not think the prophet is saying that if we fast exactly like this that God will always be available to us, healing us of all our ills. That is what the people thought would happen if they fasted in their very public and ostentatious ways. Being attentive to acts of justice, says the prophet, brings its own light into the world, and to be concerned with the oppressed brings healing to the spirit, not automatic riches and comfort from God.
"Then you shall call, and YHWH will answer; you shall cry for help, and God will say, 'Here I am' (58:9a). God is always available, attentive, attuned to those who work for justice. God is far less interested in great shows of religious practice. This writer stands in the long 300-year history of the prophets of Israel who had showed no interest whatever in public displays of piety. Justice, they said again and again, is the way to the heart of God.
So it was for this Isaianic writer and so it is with the one we call Christ, born in a manger, come to remind the world again that justice is the hallmark of God's people.