"Is not this the kind of fasting I have chosen:
to loose the chains of injustice and untie the cords of the yoke,
to set the oppressed free and break every yoke?
Is it not to share your food with the hungry and
to provide the poor wanderer with shelter—
when you see the naked, to clothe him,
and not to turn away from your own flesh and blood?
Don't have a "what do I get out of it" attitude in serving Christ. For as Jesus said, "when you have done everything you were told to do, should say, ‘We are unworthy servants; we have only done our duty.’" Luke 17:10 And though you may receive a crown, be ready to cast that crown down at the feet of Jesus. For even rewards are only of grace. We are slaves to Christ, we who have accepted Christ as our Lord, and shouldn't obsess over what we do or do not get out of it. Rather firstly have it on your mind "What does God want me to do?" and secondly "How can I edify others?"
We notice here that there were those who zealously devote themselves to fasting as a purely religious ritual to gain favor with God. But it's as if God were saying, "You idiots. You think I'm impressed just because you abstain from eating. You don't get it." We see in this passage in Isaiah that God's was not impressed with fasting itself as a religious ritual, but rather the purpose was to help the oppressed and down-troddened. In particular if you abstained from eating that means that you were to free up your servants from their work of providing the meals and all the labor associated with obtaining and preparing the food. Today of course circumstances, at least generally in the US, are different such that this area of obtaining and preparing food is not directly linked to seriously burdening someone. But more importantly we have here a general principle. We could ask, "Are there those who are seriously burdened in taking care of my needs? Is there a way I could either permanently or periodically release them from such a burden, either untying the cords of the yoke or breaking the yoke altogether?"
Secondly fasting freed up resources which could be directed to the needy. In this case the food itself or the money to prepare it could be redirected to the poor. He also gives other such applications here as housing the homeless, clothing the naked as well as providing for your own family - your own flesh and blood. For
Jas 1:27 Religion that God our Father accepts as pure and faultless is this: to look after orphans and widows in their distress and to keep oneself from being polluted by the world.So the second principle to follow is "What sacrifices can I make to free up resources (material things, money, time, energy, effort) to redirect them to the more needy and to those for whom I am responsible?"
1Ti 5:4 But if a widow has children or grandchildren, these should learn first of all to put their religion into practice by caring for their own family and so repaying their parents and grandparents, for this is pleasing to God.
1Tim 5:8 If anyone does not provide for his relatives, and especially for his immediate family, he has denied the faith and is worse than an unbeliever.