16 So let no one judge you in food or in drink, or regarding a festival or a new moon or sabbaths,
17 which are a shadow of things to come, but the substance is of Christ.
so (v.16) — in light of what Christ accomplished on the cross as outlined in vs. 13-15
let no one judge (v.16) — = sit in judgment — tense indicates an action currently happening
Nobody should use observance of the Mosaic law as a basis for judging someone’s standing before God.
food or drink (v.16) = lit. “eating and drinking” — probably refers chiefly to asceticism in general, which denied the body any pleasure, but also probably includes the dietary rules in the law
new moon (v.16) — The festival of the new moon is placed beside the Sabbath (Isaiah 1:13; Ezekiel 46:1). The day was celebrated by blowing of trumpets, special sacrifice, feasting, and religious instruction. Labor was suspended, and no national or private feasts were permitted to take place. The authorities were at great pains to fix accurately the commencement of the month denoted by the appearance of the new moon. Messengers were placed at commanding heights to watch the sky, and as soon as the new moon appeared, they hastened to communicate it to the synod, being allowed even to travel on the Sabbath for this purpose. The witnesses were assembled and examined, and when the judges were satisfied, the president pronounced the words “it is sanctified,” and the day was declared new moon. — Ephesians and Colossians in the Greek New Testament, by Kenneth S. Wuest, page 210.
substance (v.17) = reality (in contrast to the shadow of the law)
Paul was saying that the Old Testament laws regarding food and feast days were a shadow of a (then, in the Old Testament) future reality. Christ is that reality. In Christ, the “things to come” have come.