Exodus 31:13
Speak thou also unto the children of Israel, saying, Verily my sabbaths ye shall keep: for it [is] a sign between me and you throughout your generations; that [ye] may know that I [am] the L-RD that doth sanctify you.
The Sabbath is used for identification to know who one is worshipping. If the children of Israel are not keeping the Sabbath, chances are they are worshipping something other than the Holy One of Israel who created the universe and rested on the seventh day.