Isaiah 58:8 - Ellicott's Commentary On The Whole Bible

Then shall thy light... — The dawning of a new day, as in 2 Samuel 23:4; the growth as of new and healthy flesh after long illness; “righteousness,” i.e., the sentence of acquittal in the eyes of all the world, as leading the van of a triumphant march, the “glory of Jehovah” following in the rear as a protection; all these images are heaped together to paint the fulness of blessing that follows on that true renunciation of the old evil selfishness of which fasting is but a symbol and a part.