NOTES for Isa 58:6
Fasting is an old Yahwist tradition, known even from the time of the Patriarchs. Its roots go deep into history, into an antiquity about which historians today can only make inferences. The original meaning of fasting, at least among Semitic peoples, was connected with abstaining from food as a sign of mourning. This could be mourning of a personal kind, for example over deceased relatives, or public mourning, for example when an entire city might fast during a siege, an epidemic, or a natural disaster. In the second case, the fast was usually accompanied by a penitential prayer.
In the times when the second part of the Book of Isaiah was written, fasting was connected above all with contrition over one's own sins: it was presumed that the one fasting mourned over himself as over a spiritual dead man, for whom, because of his sins, there would be no place in the messianic Kingdom. Over time, fasting ceased to be experienced so personally and tragically, turning into a peculiar pious tradition that was meant to remind a person of his sinfulness.
Sometimes such piety was showy: those who fasted wanted, by their whole sad and even tragic appearance, to show how they grieved over their sins. They categorically refused food and drink if these were offered to them (fasts in those days involved complete abstinence from food and water during the day), they walked in torn clothing and with ashes sprinkled on their heads (the usual signs of mourning at that time), in short, they made it clear to those around them in every possible way that they were fasting.
The prophet, however, proposes in place of these signs of showy piety something that would become a sign of genuine repentance if a person began to act in this way: he proposes to the penitent the works of mercy that he could do. Indeed, whatever sin a person has committed, God can forgive him if his repentance is sincere.
On the other hand, a person who is truly repenting of his sin and wants to restore normal relations with God should, according to the prophet's thought, not engage in self-flagellation that neither he himself nor God needs, especially not for display, but do works that testify to his love for the neighbor who needs such love.
This is not surprising: spiritual life is, first of all, the relationships that bind a person to God and to his neighbors. Sin destroys them, and the one repenting must first of all try to restore what has been destroyed. And there is no better means for restoration than works of love and mercy. That is why God, through the mouth of His prophet, points to them as testimony of genuine repentance, which can return the sinner to God's path.