Moreover also I gave them my sabbaths, to be a sign between me and them, that they might know that I am the LORD that sanctify them.
I gave them my sabbaths, to be a sign between me and them - a kind of sacramental pledge of the covenant of adoption between God and His people. The Sabbath is specified as a sample of the whole law, to show that the law is not merely precepts, but privileges, of which the Sabbath is one of the highest. Not that the Sabbath was first instituted at Sinai, as if it were an exclusively Jewish ordinance (for it was instituted in Paradise, in the time of man's innocence, Genesis 2:2-3); but it was then more formally enacted, when, owing to the apostasy of the world from the original revelation, one people was called out to be the covenant-people of God (Deuteronomy 5:15).
That they might know that I am the Lord that sanctify them. The observance of the Sabbath contemplated by God was not a mere outward rest, but a spiritual dedication of the day to the glory of God and the good of man. Otherwise it would not be, as it is made, the pledge of universal sanctification (Exodus 31:13-17, "In the seventh is the sabbath of rest, holy (literally, holiness) to the Lord," etc.; Isaiah 58:13-14). Virtually it is said, all sanctity will flourish or decay according as this ordinance is observed in its full spirituality or not.