how is that an assumption? seriously do the words , "Thou shalt have no other gods before me," have no meaning to you. Or howabout, “You shall not make for yourself a carved image, or any likeness of anything that is in heaven above, or that is in the earth beneath, or that is in the water under the earth; you shall not bow down to them nor serve them. For I, the Lord your God, am a jealous God, visiting the iniquity of the fathers on the children to the third and fourth generations of those who hate Me, but showing mercy to thousands, to those who love Me and keep My Commandments"
If an atheist were able to keep the sabbath i see no reason why he would be worse off then some one who made the wrong choice.