When Paul says that "the reality, however, is found in Christ" he is referring to verse 9-10 when he speaks of God being found in bodily form and the "fulness" (completion of something already started) he brings to his followers. Everything we are is in Christ: He makes us complete. The actual translation of the second part of verse 17 is "...but, the body [is] of Christ" with the elipsis contrasting the second part, the real presence of God in a body, with the "shadow" or "forshadowing" referring to verses 11-16.
In verses 11-16, Paul is discussing that which foreshadowed: All the laws, traditions, and activities we did in the flesh to make us so "religious." We don't need these things anymore, at least as a substitute for the real thing.
As Oswald Chambers once said, "Beware of posing as a profound person-- God became a baby." Only Jesus is profound!
This is the miracle of the Incarnation: God became a Man. Because He became a Man, we can dispense with religious posturing and worship the only one worthy. He is the reality. He is the fulness of Deity in human form. So, this Christmas let's put aside all of our worldly cares and concerns and give our attention to the One who said on the cross "it is finished."
Pray: "God, help me to remember who we are by focussing on who He is."