Agreed. But in Ps 95 David uses this as an illustration of failing to continue to obey God and in Heb 3 it is a warning against failing to persevere in the faith. What is interesting is that Heb 4 says "some entered" what is more there is a direct appeal to the one "who has entered" God's rest.
Heb 4:8 For if Joshua had given them rest, He would not have spoken of another day after that. 9 Consequently, there remains a Sabbath rest for the people of God. 10 For the one who has entered His rest has himself also rested from his works, as God did from His.
The "rest" is past tense "has entered" in Heb 4. "HAS also rested", "HAS entered His rest". Heb 3 ends on the note of "perseverance" to continue 14 For we have become partakers of Christ if we keep the beginning of our commitment firm until the end
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The appeal for the one "who HAS entered His rest" is to remain steadfast
Heb 3:12 Take care, brothers and sisters, that there will not be in any one of you an evil, unbelieving heart that falls away from the living God. 13 But encourage one another every day, as long as it is still called “today,” so that none of you will be hardened by the deceitfulness of sin. 14 For we have become partakers of Christ if we keep the beginning of our commitment firm until the end,
Psalm 95 hearkens back to the events of Numbers 14.
Psalm 95
8 Harden not your heart, as in the provocation, and
as in the day of temptation in the wilderness:
9 When your fathers tempted me, proved me, and saw my work.
10
Forty years long was I grieved with this generation, and said, It is a people that do err in their heart, and they have not known my ways:
11 Unto whom I sware in my wrath that they should not enter into my rest.
Hebrews 3 also hearkens back to the events of Numbers 14.
Hebrews 3
15 While it is said, To day if ye will hear his voice, harden not your hearts, as in the provocation.
16 For some, when they had heard, did provoke: howbeit not all that came out of Egypt by Moses.
17 But with whom was he grieved forty years? was it not with them that had sinned, whose carcases fell in the wilderness?
18 And
to whom sware he that they should not enter into his rest, but to them that believed not?
19
So we see that they could not enter in because of unbelief.
Hebrews 4
3 For
we which have believed do enter into rest, as he said, As I have sworn in my wrath, if they shall enter into my rest: although the works were finished from the foundation of the world.
4 For he spake in a certain place of the seventh day on this wise, And God did rest the seventh day from all his works.
5 And in this place again, If they shall enter into my rest.
6 Seeing therefore
it remaineth that some must enter therein, and they
to whom it was first preached entered not in because of unbelief:
7 Again, he limiteth a certain day, saying in David, To day, after so long a time; as it is said, To day if ye will hear his voice, harden not your hearts.
8
For if Jesus had given them rest, then would he not afterward have spoken of another day.
9
There remaineth therefore a rest to the people of God.
10
For he that is entered into his rest, he also hath ceased from his own works, as God did from his.
11
Let us labour therefore to enter into that rest,
lest any man fall after the same example of unbelief.
So ... let's note a few things.
REST ... is one of the main themes of scripture. REST, biblically, ... represents a ceasing from one's labors ... as God did on the seventh day of Creation.
So ... the observation to keep the Sabbath rest (i.e. no labor/holiness) is the basis of the 4th of the 10 Commandments given to Israel in the First Covenant.
God desires to give us REST from our labors ...
"Come unto Me, all you who labor and are heavy laden, and I will give you rest. Take My yoke upon you and learn of Me, for I am meek and lowly in heart, and you will find rest for your souls. For My yoke is easy ... and My burden is light." (Matthew 11:28-30)
God desired to bring Israel into His rest (for them). However, when He commanded that they advance into the Promised Land, despite the negative reports of 10 of the 12 spies, the Israelites refused to go ... for they did not trust the Lord to give them the land. Therefore, God swore that none of that generation (which was of age to decide) would enter into the Promised Land (His rest for them). And so it was that the Israelites wandered in the wilderness for 40 years, until all of that generation had died off (which the exception of the faithful spies Caleb and Joshua). After this generation had thus died off, the younger Israelites were allowed by God to enter the Promised Land under Joshua.
This rather pivotal event in Israel's history is referred to numerous times in the writings of those that came after that time. Psalm 95 reflects upon it ... and so, Hebrews 3 and 4. The event ... and the memorial writings highlight the significance of FAITH in God by His people. And those events were, in some sense, a shadow of what was yet to come. For, ultimately, God would offer the individual members of humanity the opportunity to TRUST in Him ... to provide rest for them (i.e. salvation). As in the case of the OT Israelites, that rest is not something that we can work for, ... rather it is something which we have to place our FAITH in God to provide.
Thus Paul writes centuries later that God's rest for believers (i.e. salvation) is OF God's GRACE, ... through the FAITH which He gives to us, ... and emphatically ... NOT OF WORKS. It is something that we have to trust God to provide to us. We can't work to OBTAIN it ... and we can't work to KEEP it (Galatians 3).
I would submit that the seventh day Sabbath is, itself, also a shadow ... of the fulfillment of REST God desires to give to us. It is a reminder (God is fond of these) ... that it is God's intention to give us His rest ... if we have but the FAITH to Trust in Him to do so.
As to the significance of Sabbath day rest on the seventh day, ... I think that God chose the seventh day ... because that is the day upon which He rested from His labors. It is clearly important for us to have one day's rest out of each seven, and if one wished to be adherent to the Law given to Israel in the first Covenant, ... then the seventh day would be the day that we should REST (i.e not work). But Jesus Himself demonstrated that slavish adherence to seventh day traditions had a measure of flexibility. He allowed His disciples to glean on the Sabbath, He healed on the Sabbath, and He said the "It is good ... to do good ... on the Sabbath", whether that involves work or not. He rightly said that He was Lord of the Sabbath (i.e. He dictated the terms of its observance) ... and that ultimately, from His standpoint, that "The Sabbath was made for man, and not man for the Sabbath." I interpret that to mean that the Sabbath (i.e. rest of God) should be observed to the benefit of humanity ... and not so rigidly ... that it loses its benefit.
When one celebrates the Sabbath should not be a cause for divisions among believers. To this end, Paul said ...
Romans 14
4 "Who art thou that judges another man's servant? to his own master he stands or falls. Yea, he shall be held up: for God is able to make him stand.
5
One man esteems one day above another: another esteems every day alike. Let every man be fully persuaded in his own mind.
6 He that regards the day, regards it unto the Lord; and he that regards not the day, to the Lord he doth not regard it."
We are not to judge one another regarding the days that we choose to esteem before God. Such a thing simply is not significant enough to break the fellowship of Christ's followers.