You fast with contention and strife to strike viciously with your fist. You cannot fast as you do today and have your voice be heard on high.
Here we have, I. The displeasure which these hypocrites conceived against God for not accepting the services which they themselves had a mighty opinion of (Isa 58:3): Wherefore have we fasted, say they, and thou seest not? Thus they went in the way of Cain, who was angry at God, and resented it as a gross affront that his offering was not accepted.
Commenting on Isaiah 58:3-7
Behold, ye fast for strife and debate,.... Brawling with their servants for not doing work enough; or quarrelling with their debtors for not paying their debts; or the main of their religion lay in contentions and strifes about words, vain hot disputations about rites and ceremonies in worship, as is well known to have been the case of the reformed churches: and to smite with...
ye shall not fast--rather, "ye do not fast at this time, so as to make your voice to be heard on high," that is, in heaven; your aim in fasting is strife, not to gain the ear of God [MAURER] (Kg1 21:9, Kg1 21:12-13).