In the book of Isaiah, many precious things are hidden. Two of these things are the keeping of the Sabbath and fasting.
To keep the Sabbath is to be "fired" and replaced by Christ so that we may get into Him and rest in Him for eternity. Therefore, to keep the Sabbath means that we stop our work, that we cease from our intentions, and that we are fired and replaced. This is to be crucified with Christ with the result that it is no longer we who live, but Christ lives in us (Gal. 2:20).
In the early days I was taught that to fast is to restrict ourselves and our lusts. This teaching is not wrong, but it surely is not complete and it may lead to asceticism. The real meaning of fasting according to the Bible is to cease from the taste for anything other than Christ. When we fast in this way, we have an appetite only for Christ and we do not eat anything other than Christ.
Isaiah's aspiration is that we all would rest in Christ and taste Christ all the time. In the New Jerusalem for eternity, we will have a life of resting and fasting. Our unique food will be Christ, the embodiment of the Triune God, as the tree of life. Furthermore, having been fully fired, we will not work but will be at rest. This will be the situation of everyone in the New Jerusalem.
Up to chapter fifty-nine, the book of Isaiah consummates in the keeping of the Sabbath and fasting. Through all His processes, Christ as the embodiment of the Triune God has accomplished God's full redemption, and now He is applying His complete salvation to His believers. Although many Christians are preaching the gospel, few people receive this preaching. Because people are busy working and eating, few have any room in their heart, any capacity in their being, for Christ. Thus, they all need to rest, to keep the Sabbath; they also need to fast. By resting and fasting we can partake of all that the processed Christ has accomplished for us. In totality, what He is and has accomplished is just the divine water, which is the consummated Spirit as the consummation of the Triune God for us to drink and enjoy.
I can testify that before I was saved, I was a busy and ambitious young man. But when I heard the gospel and received the Lord Jesus, I immediately began to keep the Sabbath and to fast. Now, more than sixty years later, I still like to rest and to fast. I want to be "fired" again and replaced by Christ more and more, resting in Him and fasting from every taste other than Him. According to the divine revelation in the Scriptures, we all need to learn to keep the Sabbath and to fast.
Let us now go on to chapters fifty-nine and sixty, which reveal Christ, the Servant of Jehovah, as the Redeemer to save Jacob from their sins and iniquities and to become Israel's light and glory forever. Having redeemed us, Christ is now our Savior. On the negative side, He is saving us from our sins and iniquities; on the positive side, He is our light and glory.