Exodus 31:13

It is a sign between me and you. — Circumcision had been given as a covenant sign to Abraham and his descendants (Genesis 17:9); but its adoption by many of the heathen nations had rendered it no longer a distinguishing mark by which God’s people could be certainly known from others. Thus a new “sign” was needed. The observance of one day in seven as a day of holy rest became henceforth the distinguishing sign, and proved effectual. It was not likely to be adopted, and in point of fact was not adopted, by any of the heathen. We find it in the latest time of the Jewish nation still regarded as the special mark and badge of a Jew (Juv. Sat. vi. 159, 14:96; Mart. Epig. 4:4, 50:7, &c.).