The prophet Isaiah’s complaint about inadequate fasting reminds us of the previous week’s reading from Micah, for good reason. Again a sort of religiosity is being substituted for true religion by those being criticized, and again economics and justice are at the heart of it all.
Scholars see the last ten chapters of Isaiah - the “Third Isaiah” - as written after the exile, in a time of restoration and hope, but of uncertainty and fragility as well. The addressees are serious (58:2) about their “spiritual practices” as some now say (not a concept that would fly for Isaiah I fear, but anyway). Yet they are disappointed with the results (58:3).
One of the reasons that this group now leans on fasting, rather than on sacrifices (cf. Micah) to twist the divine arm, is that there may not even have been a Temple to sacrifice in, after the destruction of 587 BC (see 58:12). So they had already leant into a more personal “spiritual practice,” away from institutional externals, but this seems to have been part of the problem.
As for Micah, preachers may be tempted to set up false oppositions, between actual fasting (yes, Lent is getting closer) and alternative forms of action that are worthy but don’t involve any real self-denial. “Don’t worry about fasting, do something else instead.” This isn’t really where Isaiah (or this Isaiah - see above) is coming from.
One of the reasons modern interpretation of these “anti-cultic” passages in the prophets and Psalms often goes off the rails in a sort of either/or way is because our assumptions about religion are so different from theirs. We see religiosity being attacked, and go quickly to modern experience, where religion is a choice rather than an assumption. In that case we imagine religiosity as a problem, rather than a shared reality. From there we may go to “don’t be religious, be an activist.” For Isaiah however, everyone is “religious.” There isn’t really anywhere in the Bible where people are attacked for being religious, because everybody is religious; bad religion however is often a problem.
In this case, bad religion doesn’t mean merely external observances rather than interior faith (that’s a Reformation era perspective), but almost the opposite; bad religion means internalizing and instrumentalizing the meaning of fasting, so it loses the real, social, purpose and power that Isaiah wants to defend. Fasting is not a spiritual practice intended to persuade God to accede to human demands as they think, it is a material practice intended to fulfill God’s demands for humans.
So we would be wrong to assume that God doesn’t want fasting and seeks social action instead. When the prophet says, on God’s behalf, “is not this the fast I choose?” (v.5) he is not engaging in metaphors, transferring the meaning of “fasting” from ascetic bodily practice to social action or anything else. He still means fasting, although new weight and scope is added. Note that hunger - not eating - is still central here (“Is it not to share your bread with the hungry?”). Isaiah still envisages people with relative abundance choosing non-consumption - fasting - not for personal fulfillment, but as a response to meeting others’ need.
Food cannot merely be a symbol when people are hungry. Food is also not an infinite resource, but must be shared. What the privileged consume does matter - and changes to material action, including by fasting in the form of sharing, are inescapable. And of other forms of material practice, that put the needs of all ahead of the self-fulfillment of some, are now added; so Isaiah hasn’t spiritualized or metaphorized fasting, but extended it. So fasting (and hence sharing) is envisaged in housing and clothing as well.
Isaiah, then, does want fasting - literal, serious fasting - but with a focus on the needs of all the people. This is social action, but it’s actually pious ritual practice too. Isaiah - like Micah - is not preferring the social over the religious, but reminding his hearers that true religion always includes the social even in its ritual practices. To spiritualize that practice is not the solution. It is even tempting to say that Isaiah advocates being (properly) religious, not (merely) spiritual.