"The observance of the Sabbath proclaimed on Sinai by an Israelite outweighs all other commandments. And from the point of view that the Sabbath was established as a token between God and his people (Exod. 31. 13) one is justified in saying that it is not right and proper for a non-Jew to observe that Sabbath; it is the expression of a relation so intimate that the intrusion of a stranger would be resented."