"Thy righteousness shall go before thee." What does this mean? I believe that this is related to the first promise, that the light of the Lord will be with us. His light will shine in our countenances. People, at some level, are aware of the light in a person's countenance. Some people are particularly radiant. Their good works, their righteousness goes before them. The Savior is a good example of this. He could not go anywhere without people coming to see Him, usually to be healed. His works were known throughout Israel. People will know and hear of our goodness, our righteousness, before they meet us, if we follow the teachings and commandments of the Savior. This promise also has a different meaning. The Lord not only is the source of our righteousness, He is Righteousness. It is He who goes before our face, protecting us from onslaughts by the enemy.
"The glory of the Lord shall be thy rereward." To understand this promise, we have to first understand what the word "rereward" means. This an old military term that refers to the guard at the rear of an army. Armies not only scouted ahead to watch for enemies, good armies also had soldiers on guard at the back of the army to watch out for flank attacks from enemies. The Lord will be both in front and behind us, protecting and guiding us. Such is the promise of faithful fasting.
"Then shalt thou call, and the Lord shall answer." This is a statement. Isaiah does not state, "If you call, the Lord might answer." We are promised that the Lord will answer. The Lord always answers our prayers, even if the answer is "No" or "Wait." He doesn't look at the caller-ID and say, "Oh no, Jared's calling again. I'll pretend I'm not here." He is there for us.
"Thou shalt cry, and he shall say, Here I am." This is the same as the last promise. However, when we cry, when we are hurt, or sad, or ashamed of what we have done, the Lord matches our tears with His own. When we cry out, when we are lost and scared in a dark forest of despair, the Lord hears and responds. Are we listening for His voice?