Paul goes on to point out that "we have been delivered from the law" by Jesus Christ, who fulfilled the law in His own person, thus doing what Israel, to whom the law was given, was unable to do (v. 6). In and through Christ we have "died to what we were held by," namely sin and death, in order to "serve in the newness of the Spirit and not in the oldness of the letter" (v. 6). What is important about this is not that what we do no longer matters, though it change significantly, but how we go about doing it and why we do it totally changes. The relationship between faith and works, to put it another way, is reversed, with faith gaining the upper hand. We are only able to do it because the Holy Spirit, who is the mode of Christ's resurrection presence in us and among us, to use an Eastern Christian term, energizes us with God's power, which is power unto (to sound a bit archaic) eternal life.
As Christ himself taught us, we cannot serve two masters. Echoing this teaching in chapter six of Romans, St. Paul points out that you either serve sin or serve God and that only by serving God do you experience true freedom. This certainly has a lot of bearing on fasting in its various forms. After all practicing the spiritual disciplines "is not bondage. It is a rejection of bondage, a pursuit of holiness while gaining freedom from legalism and sinful passions. The freedom of grace is known through the discipline of being bound to God the Father, to Christ, and to the Holy Spirit. Christ fulfilled the law with its demands that we might be free in Him, justified by faith, to live fruitful and righteous lives, obeying the truth." (Orthodox Study Bible Introduction to The Epistle of St. Paul to the Galatians).