Picking up where Amos left off, Isaiah blisters the Religious Duty Party who think that they have God cornered since they fast and pray and perform their appropriate religious functions. They have the audacity to ask God why they fasted and He hasn't paid attention to their pain. Like a kid who quits crying when you leave the room and starts it back up when you enter it again, they were fasting merely to get their way.
But what is God's chosen fast? First, let me buffer this (ever so slightly) by pointing out that Jesus is for fasting. Don't get that wrong.
But, if we fast and don't loose the bonds of wickedness and undo the straps of the yoke to let the oppressed go free and share our bread with the hungry and see someone naked and clothe him, then our fasting doesn't do us much good.
Religious functions are not enough. Doing the right thing has to accompany them. And the only way a person does the right thing is through the transformational power of the Holy Spirit, sent by Jesus Christ and the Father, to take up residence inside of everyone who trusts in Truth of the Gospel.