John Calvin Commentary Isaiah 58:10

"and if thou draw out thy soul to the hungry, and satisfy the afflicted soul: then shall thy light rise in darkness, and thine obscurity be as the noonday;" — Isaiah 58:10 (ASV)

If you pour out your soul to the hungry. He goes on to recommend the duties of that love which we owe to one another. The sum of the whole discourse is this: men serve God in vain if they only offer Him trivial and bare ceremonies. This is not the right and proper worship of God, who rigidly commands and requires us to lead an upright and innocent life with our neighbors, willingly to give ourselves and our labors to them, and to be ready to assist them readily and cheerfully whenever it is necessary.

We should observe the two parts of this duty which the Prophet has expressly described: first, he recommends to us the feeling of mercy and kindness; and second, he exhorts us to the work itself and its effect. It would not be enough to perform acts of kindness towards men if our disposition towards them were not warm and affectionate. If I give all my goods to the poor, says Paul, and have not love, I am nothing (1 Corinthians 13:3). To “pour out the soul,” therefore, is nothing other than to mourn their distresses and to be as much affected by their poverty as if we ourselves endured it. On the other hand, all who are limited and devoted to themselves are said to have a hard and seared heart, to shut up their bowels (1 John 3:17), and to restrain their feelings. Another translation given by some commentators, “If you offer your soul,” is unworthy of notice.

Your light shall arise in darkness. Again, there follows the same promise, and under the same figure or metaphor. By “darkness” he denotes adversity, and by “light” prosperity, as if he had said to the people, “The Lord will cause all the miseries by which you are now oppressed to cease, and sudden prosperity will spring up.” He shows, therefore, that there is no reason for them to blame God for punishing them so severely, for they would immediately be delivered and enjoy prosperity if they sincerely worshipped and obeyed God.