Vincent's NT Word Studies
17. Which are. Explanatory. Seeing they are. Referring to all the particulars of ver. 16.Shadow of things to come. Shadow, not sketch or outline, as is shown by body following. The Mosaic ritual system was to the great verities of the Gospel what the shadow is to the man, a mere general type or resemblance.
The body is Christ's. The substance belongs to the Christian economy. It is derived from Christ, and can be realized only through union with Him.
Robertson's NT Word Studies
2:17 {A shadow} (skia). Old word, opposed to substance (swma, body). In #Heb 10:1 skia is distinguished from eikwn (picture), but here from swma (body, substance). The swma (body) casts the skia (shadow) and so belongs to Christ (cristou, genitive case).