Eze 20:11 I gave them my statutes and made known to them my rules, by which, if a person does them, he shall live.
Eze 20:12 Moreover, I gave them my Sabbaths, as a sign between me and them, that they might know that I am the LORD who sanctifies them.
Eze 20:13 But the house of Israel rebelled against me in the wilderness. They did not walk in my statutes but rejected my rules, by which, if a person does them, he shall live; and my Sabbaths they greatly profaned. “Then I said I would pour out my wrath upon them in the wilderness, to make a full end of them.
Do you notice something unusual about what is said above? Adventists talk about the Sabbath being part of the "moral" law. Apart from it being a memorial, and a sign, and it being an appointed time, and having sacrifices, it is stated to be in addition to the laws by which if one does them they will live.
Also related to something you indicated earlier, I don't know if the Scriptures ever say explicitly that the Sabbath is the sign of the covenant, but it does say it is a sign, and is a covenant.
According to the scriptures in the old and new covenant Gods' 4th commandment is one of Gods'10 commandments that give us the knowledge of good (moral right doing and evil (moral wrong doing); sin (moral wrong doing) and righteousness (moral right doing) *Romans 3:20; Romans 7:7; 1 John 3:4; Psalms 119:172 and according to James if we break any one of them we stand guilty before God of sin *James 2:10-11.
Gods' 4th commandment says...
Exodus 20:8-11 [8], Remember the SABBATH DAY, to KEEP IT HOLY. <Why?> Because God made it Holy for mankind and commands us to keep it as a Holy day) [9], Six days shalt thou labor, and do all thy work: [10], But the SEVENTH DAY IS THE SABBATH of the LORD thy God: in it thou shalt not do any work, thou, nor thy son, nor thy daughter, thy manservant, nor thy maidservant, nor thy cattle, nor thy stranger that is within thy gates: <WHY> [11], For in six days the LORD made heaven and earth, the sea, and all that in them is, and rested the SEVENTH DAY: wherefore the LORD BLESSED THE SABBATH, and HALLOWED IT.
Notice: What the commandment actually says. It starts of "REMEMBER" the Sabbath day v8, it defines the Sabbath day as the "seventh day" of the week v10 then gives the reason why the Sabbath must be kept in v11 where it says For in six days the LORD made heaven and earth, the sea, and all that in them is, and rested the SEVENTH DAY: wherefore the LORD BLESSED THE SABBATH, and HALLOWED IT. This is pointing BACKWARDS to Genesis 2:1-3. Lets compare Exodus 20:11 to what it is quoting from in Genesis 2:1-3 and see if they match up.
We are told in the commandment...
Exodus 20:8 [8], REMEMBER THE SABBATH DAY, TO KEEP IT HOLY.
Memorial pointing backwards while the reason is given in v11
[11], For in six days the LORD made heaven and earth, the sea, and all that in them is, and RESTED THE SEVENTH DAY: wherefore the LORD BLESSED THE SABBATH, and HALLOWED IT.
This links directly backwards to the origin of the Sabbath Genesis 2:1-3 which uses similar wording..
Genesis 2:1-3[1], Thus the heavens and the earth were finished, and all the host of them.
[2], And ON THE SEVENTH DAY God ended his work which he had made; and he RESTED ON THE SEVENTH DAY from all his work which he had made. [3], And GOD BLESSED THE SEVENTH DAY, and made it HOLY <the 7th day>: because that in it he had rested from all his work which God created and made.
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Note: Did you notice the links between God's 4th commandment and Genesis 2:1-3? Unlike the appointed times in the annual Feast days and the annual sabbaths which are many and days of holy convocation God's 4th commandment "seventh day" Sabbath of the 10 commandments points BACKWARD to the FINISHED work of creation not forward to things to come like the appointed Feast days. All the annual appointed feast days were shadows of things to come fulfilled in Christ and God's plan of salvation under the new covenant. This is impossible for God's "seventh day" Sabbath because it it points backwards as a memorial of creation. When God made the Sabbath for mankind there was no sin, no shadow laws, no Jew, no Moses, no Israel and no plan of salvation given because it was not needed because there was no sin. Therefore it is impossible for God's "seventh day" Sabbath of creation to be a shadow of things to come when it points backwards to the finished work of creation.
Gods 4th commandment Sabbath written in Ezekiel 20:11-13 therefore is not an addition. "More ever" or " Further more" in Ezekiel 20:12 is following on from what God has already just said in Ezekiel 20:11. God is saying that "Further more to giving His people His laws, He gave them His Sabbaths,
as a sign between me and them, that they might know that I am the LORD who sanctifies them.
Hope this is helpful.