NOTES for Isa 58:1-9

Fasting, as a religious practice, was known both to Yahwism and to Judaism. Still, neither Yahwism nor Judaism knew multi-day fasts like those Christians of the Middle Ages came to practice. The original meaning of a fast was mourning, usually for a deceased relative, and such a mourning fast could usually last from one to three days. The fast was ordinarily complete: those fasting ate and drank nothing from the evening of the day before the fast until the evening of the fast day itself. In addition, whole cities or villages could take on fasts in the event of some disaster, especially one that was known (often from a prophet's words) to have been allowed by God because of the inhabitants' sins. But such fasts were one-time fasts, and those who fasted took them on for a specific occasion each time.

In the post-exilic period, when the religious life of the Jewish people was already unthinkable outside the synagogue, the practice arose of regular one-day fasts, usually twice a week. These were no longer connected with specific events (in such cases people fasted additionally), but were regarded as a kind of spiritual exercise, and performing it was a religious obligation for every believing Jew. These fasts were understood to be connected with the sins a person fasting might have committed (perhaps without even knowing it), but, as usually happens in such cases, they became a simple formality, a kind of "sacrifice" to God that the fasting person offered Him by refusing water and food for a whole day. Few thought to ask whether God needed such a fast at all. Abstaining from food and drink, it was thought, was supposed to make the fasting person leave the ordinary rut of everyday life, remember God and his neighbor, and experience anew and vividly what a person tends to forget in the bustle of daily routine. In practice, however, abstinence itself became part of the everyday religious routine, as familiar as the routine itself.

In such a situation the prophet had to remind those who fasted that fasting is not simply a religious obligation and not a sacrifice to God, who does not need exercises in abstinence in themselves, but a way to shift a person's attention from himself to God and to his neighbors. If a fast does not fulfill this function, God does not need it and it does the person no good.