An interesting and significant term is used in Exodus 31:13, 17: The Sabbath is a sign, not a mark. Bible usage shows that a sign is something voluntarily accepted, whereas a mark is placed on a person against his will. The Sabbath is a special sign of a special covenant between God and His people. In the wilderness, His people were the Israelites; today, it is the Israel of God, the church of God (Galatians 6:16).
A sign can identify a person's occupation or an individual's or group's purpose. Signs can give directions (like traffic signs), or they can bring people together with shared interests and a common purpose (like flags). A sign can be the pledge of mutual fidelity and commitment (as in putting a hand over one's heart). Organizations use signs to designate membership so that members can recognize each other (as with secret handshakes).
In the church's case, the Sabbath serves as an external and visible bond that simultaneously unites and sanctifies us from everyone else. Almost everybody else keeps Sunday or nothing. Through the Sabbath, the Christian knows that God is sanctifying him. Everyone who has kept both Sabbath and Sunday knows this. Sunday sets no one apart from this world because so many in this world observe it.
God has a purpose He is working out. If the only reason He created the Sabbath were because we need physical rest, any old time would do. Ultimately, however, the real sign appears in how and why one keeps the Sabbath. Thus, God made a specific period of time special so that He could meet with His people during that sanctified period to help them become even more different for their benefit.
What is His major goal in doing this? He educates His people in His way of life. In part, He prepares His people to witness for Him by this means. As an analogy, suppose a basketball coach told his players, "Come to the gym at 8 AM Monday, and I will teach you how my team plays ball." Yet, what if some players went to a different gym at a different time with a different coach?
Players on a team who take the game seriously begin to take on the qualities and philosophy of their coach. People who involve themselves deeply in athletics say they can often tell by a player's characteristics that he has trained under a certain coach. They remark that he has the John Wooden or John Thompson "way" about him. What has happened is that the player has taken on the "sign" of his coach, and it has "sanctified" him from other players who were not trained by that particular coach.
The Sabbath was created because it both enhances and protects a Christian's relationship with God. It also provides a witness to God, to the person keeping it, and to the world. It exists to help keep a Christian pointed in the right direction and in a proper frame of mind, and it provides him with the right knowledge and understanding to negotiate the way to God's Kingdom.
John W. Ritenbaugh
The Fourth Commandment