“Behold, in the day of your fast you find pleasure,
And you oppress all those who labour on your behalf,
Behold you fast for strife and contention,
And to smite with the fist of wickedness.
You do not fast as you do this day
So as to make your voice heard on high.”
God replies to their claims. Their fasting is hypocritical, for while engaging in it they go out of their way to find their own pleasure. It is half-hearted. This suggests that it was often a formal fast without being too strictly observed, or without being too demanding. Furthermore their claims to exercise fair judgments is not true. Instead their behaviour is abominable. Even while outwardly fasting and appearing humble they oppress their workers, they involve themselves in strife and court action in every aspect of life, and they behave with unreasonable and sin-inspired violence. They are as far from ‘truly just' as it is possible to be. Indeed they use the time of their fast for these very purposes.
Thus any seeming merit in their fasting is lost in view of their attitude to life and its resulting behaviour. This kind of fasting will not result in their voice being heard by God.
Fasting is a feature of religion in any age. Where it results in a genuine seeking after God Himself it can be beneficial. But it can easily deteriorate into being seen as a means of putting God under an obligation so that He has to respond, or of impressing men with one's holiness.
There is no need to see these as official fasts. Indeed the idea is that they occurred continually in the every day course of life, as did their sins that accompanied them. It may well be that such regular days of fasting had become the fashion in Isaiah's troubled times, due to the outward threat of Assyria, as they did later with the Pharisees.