Your quote from Heb 3 - The old generation of Israel were not able to enter into his rest, because of their unbelief, God caused them to go around in circles in the wilderness, until they all died of old age, they were still his people, and retained their promise of an eternal inheritance.
Heb 3:7 Wherefore (as the Holy Ghost saith, To day if ye will hear his voice,
Heb 3:8 Harden not your hearts, as in the provocation, in the day of temptation in the wilderness:
Heb 3:9 When your fathers tempted me, proved me, and saw my works forty years.
Heb 3:10 Wherefore I was grieved with that generation, and said, They do alway err in their heart; and they have not known my ways.
Heb 3:11 So I sware in my wrath, They shall not enter into my rest.)
Heb 3:12 Take heed, brethren, lest there be in any of you an evil heart of unbelief, in departing from the living God.
Heb 3:13 But exhort one another daily, while it is called To day; lest any of you be hardened through the deceitfulness of sin.
There is a warning to the saints to not fall victim to the same attitude of the disobedient Jews which if they do then they have departed from the living God so how are they saved if they departed from the living God which is by the deceitfulness of sins.
So if they fall victim to the same fate as those disobedient in the wilderness then they would not enter in to God's rest which would be the spiritual realm a spiritual rest.
And it says they departed from the living God by the deceitfulness of sins then the Jews who were disobedient departed from the living God so how were they saved.
Isa 59:1 Behold, the LORD'S hand is not shortened, that it cannot save; neither his ear heavy, that it cannot hear:
Isa 59:2 But your iniquities have separated between you and your God, and your sins have hid his face from you, that he will not hear.
2Pe 2:20 For if after they have escaped the pollutions of the world through the knowledge of the Lord and Saviour Jesus Christ, they are again entangled therein, and overcome, the latter end is worse with them than the beginning.
2Pe 2:21 For it had been better for them not to have known the way of righteousness, than, after they have known it, to turn from the holy commandment delivered unto them.
2Pe 2:22 But it is happened unto them according to the true proverb, The dog is turned to his own vomit again; and the sow that was washed to her wallowing in the mire.
Rom 11:19 Thou wilt say then, The branches were broken off, that I might be graffed in.
Rom 11:20 Well; because of unbelief they were broken off, and thou standest by faith. Be not highminded, but fear:
Rom 11:21 For if God spared not the natural branches, take heed lest he also spare not thee.
Rom 11:22 Behold therefore the goodness and severity of God: on them which fell, severity; but toward thee, goodness, if thou continue in his goodness: otherwise thou also shalt be cut off.
Any person that has been saved and then goes back to sinning is like a sow that was cleaned going back to their wallowing in the mud, and it would of been better if they had not known the truth than to depart from it.
The Jews who were disobedient were broken off and were not part of Israel anymore.
The Bible says for the saints to not be highminded but fear lest God not spare them either for if they do not continue in the goodness of God as the Jews in the Old Testament then they would also be cut off.
