This special covenant—strategically placed by Moses between information on the building of the Tabernacle (a type of the church) and the Golden Calf incident (brazen idolatry)—creates a special sign of the Sabbath between God and His people. Generally, a sign identifies. It communicates the purpose of or gives directions to a person or place. Signs bring people together with shared interests and common goals. A sign can function as a pledge of mutual fidelity and commitment. Organizations use signs to designate membership, allowing members to recognize each other.
The Sabbath serves as an external and visible bond that unites God's people, and at the same time it sanctifies them from almost everyone else. Almost everyone in the Western world keeps Sunday or nothing. By the Sabbath, the true covenant-keeper knows that God is sanctifying him. Anybody who has kept both Sunday and Sabbath knows this: Sunday sets no one apart from this world.
If He created the Sabbath only because we need to rest physically, any old time would do, but ultimately, how and why we keep the Sabbath is what becomes the real sign. God is working out a purpose. He has invested a tremendous amount in us in the creation and in the death of His Son. The Sabbath serves as a major means by which He protects that investment. He made a specific period of time special so He can meet with His people and take major steps to make them different—holy.