How do we know that the Sabbath was instituted for all humanity in every age, not only for the Israelite nation? Gen. 2:1-3.
In his Creation account, Moses mentions that, after concluding His six days of Creation, God rested on the Sabbath (Gen. 2:1-3). The Lord repeats this fact in the fourth commandment (Exod. 20:8-11). Gordon J. Wenham states: "The seventh day is the very first thing to be hallowed in Scripture, to acquire that special status that properly belongs to God alone. In this way Genesis emphasizes the sacredness of the Sabbath. Coupled with the threefold reference to God resting from all his work on that day, these verses give the clearest of hints of how man created in the divine image should conduct himself on the seventh day."--Word Biblical Commentary, Gen. 1-15 (Waco, Tex.: Word Books, 1987), p. 36.
What is different about the Genesis reference to the seventh day in Creation and the references to the other six days? Gen. 1:5, 8, 13, 19, 23, 31; Gen. 2:2, 3.
Unlike the other six, the seventh Creation day is not designated by "evening and morning." Some scholars claim that the six days were set periods, whereas the seventh was open-ended. Thus, they suggest the Sabbath was pre-Fall time, to be restored when sin and sinners are no more. This overlooks three important facts: (1) The seventh day is called a day (Hebrew yom; Gen. 2:2), as are the previous six days (Gen. 1:5-31). (2) The last day of Creation week is called the "seventh." (3) The fourth commandment equates the seven as equal parts of one week (Exod. 20:8-11). Therefore, Creation Sabbath was not an extended period of time anymore than were the previous six Creation days.
The word for day in Hebrew (yom) always means a 24-hour period when used with the adjectival numerals "first," "second," "third," etc. Hence, Genesis 1 speaks of Creation in six literal days. The normal linguistic meaning of "seventh day" in Genesis 2:2 is the same as the meaning of the previous six days. Unlike the month and the year, which are marked off respectively by the moon- and sun-rotation cycles, there is no natural phenomenon to mark off the week. The week originates from Creation week.
What is the relevance for the Sabbath doctrine that the weekly cycle has never been changed?