Because of the New Creation which brought with it a new memorial!
The seventh day was a memorial of the original creation that fell (Genesis 2:2-3, Exodus 20:8).
But it was also a sign of redemption. The parallel to Exodus 20:8 is Deuteronomy 5:12-15 which gives a second reason outside of creation—and that’s redemption. It was a sign for Israel of their freedom from slavery in Egypt.
So sabbath is ultimately about creation, rest and redemption. In 2 Corinthians 5:16-17 Paul tells us the new creation is here already and that we are new creatures in Christ.
With that being the case, we aren’t in the old creation anymore with the old creation memorial. We’re in the new creation with a new memorial—the day Jesus accomplished the work of our redemption.
Hebrews 4:1-11 explains for us that just like God worked in creation and rested the seventh day. So Jesus worked for our redemption, inaugurating a new creation, and entered into His mediatorial rest for His people, ceasing from work—which was the first day. This establishing a new memorial of the new creation. The Christian Church meets every 7 days from that new memorial.
The Exodus account teaches us the creational aspect to the sabbath and Deuteronomy shows us the redemptive aspect. The old creation, of which the seventh day was a memorial sign, was a type and shadow of the New Creation that was inaugurated with the resurrection of Christ. Deliverance from slavery in Egypt was a type and shadow of our redemption and freedom from slavery to sin, which Christ delivers us from through His finished work of atonement. And the substance of all of it—both old and new—is Jesus.
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