ישעיהו, פרק נ״ח, פסוק ד׳

Fasting is intended to be a time of spiritual elevation, but it can easily deteriorate into a display of violence and hypocrisy. Instead of physical deprivation inspiring true repentance, it often breeds social friction. The people are accused of using their fast days for arguments and disputes. This occurs because the fast brings a break from regular work. People gather together, inevitably crossing paths with their enemies, and the natural irritability that comes from a lack of food quickly ignites quarrels [מצודת דוד, ביאור שטיינזלץ]. There is also a striking contrast in how the people approach different types of assemblies. When called to gather for the sake of God, they view it as a heavy burden. Yet, when they come together to argue and confront one another, they do so with deep enthusiasm and a willing spirit [מלבי״ם].

These verbal disputes rapidly escalate into physical violence. The fighting turns into striking one another with a wicked fist, understood either as a tightly closed hand [מצודת ציון] or as using a hard object, such as a clod of dirt, as a weapon [אבן עזרא].

Because of this behavior, the prophet declares that they cannot continue fasting right now, or in this current manner, and expect their voices to be heard on high [שד״ל, אבן עזרא]. The primary approach among commentators is that the people are simply not fasting properly. A genuine fast requires a broken heart and a sincere internal transformation so that prayers can reach the heavens [מצודת ציון]. However, the people treat the day merely as an exercise in avoiding food. Their loud prayers and cries toward God are entirely deceitful. They shout as though they are pouring out their souls, but God looks directly at their hearts and completely rejects their fast [רד״ק, רש״י, ביאור שטיינזלץ, צאינה וראינה].