Therefore let no one act as your judge in regard to food or drink or in respect to a festival or a new moon or a Sabbath day—things which are a mere shadow of what is to come; but the substance belongs to Christ. (Colossians 2:16--17)
Legalism is the religion of human achievement. It argues that spirituality is based on Christ plus human works. It makes conformity to manmade rules the measure of spirituality. Believers, however, are complete in Christ, who has provided complete salvation, forgiveness, and victory. Therefore, Paul tells the Colossians, let no one act as your judge. Do not sacrifice your freedom in Christ for a set of manmade rules. Inasmuch as "Christ is the end of the law for righteousness to everyone who believes" (Rom. 10:4), to become entangled again in a legalistic system is pointless and harmful. Paul reminded the Galatians, who were also beguiled by legalism, "It was for freedom that Christ set us free; therefore keep standing firm and do not be subject again to a yoke of slavery" (Gal. 5:1).
Legalism is useless because it cannot restrain the flesh. It is also dangerously deceptive, because inwardly rebellious and disobedient Christians, or even nonChristians, can conform to a set of external performance standards or rituals.
That Christians not be intimidated by legalism was Paul's constant concern. He commanded Titus not to pay attention to "Jewish myths and commandments of men who turn away from the truth," because "to the pure, all things are pure; but to those who are defiled and unbelieving, nothing is pure, but both their mind and their conscience are defiled" (Titus 1:14--15). Romans 14--15 and 1 Corinthians 8--10 also discuss Christian liberty and the only legitimate reason for restraining it: to protect a weaker Christian brother or sister.
The false teachers were telling the Colossians that it was not enough to have Christ; they also needed to keep the Jewish ceremonial law. The false teachers' prohibitions about food and drink were probably based on the Old Testament dietary laws (cf. Lev. 11). Those laws were given to mark Israel as God's distinct people and to discourage them from intermingling with the surrounding nations.
Because the Colossians were under the New Covenant, the dietary laws of the Old Covenant were no longer in force. Jesus made that clear (cf. Mark 7:14--19).
Paul reminded the Romans that "the kingdom of God is not eating and drinking, but righteousness and peace and joy in the Holy Spirit" (Rom. 14:17). That the dietary laws are no longer in force was illustrated by Peter's vision (Acts 10:9--16) and formally ratified by the Jerusalem Council (Acts 15:28--29).
A festival was one of the annual Jewish celebrations, such as Passover, Pentecost, the Feast of Tabernacles, or the Feast of Lights (cf. Lev. 23). Sacrifices were also offered on the new moon, or first day of the month (Num. 28:11--14).
Contrary to the claims of some today, Christians are not required to worship on the Sabbath day. It, like the other Old Covenant holy days Paul mentions, is not binding under the New Covenant. There is convincing evidence for that in Scripture.