In Hebrews 4:9, believers in Christ were told that there remains a Sabbath rest unto the people of God, and in v.11 that they were to labor to enter into that rest.
Hebrews was written for Jewish believers in tribulation.
What was it/ is it, and how were they/ are we to labor to enter into it?
The Jewish believers are urged to take heed from departing from the living God due to unbelief.
They were urged to labor to enter into the promise of rest from their own works, not to fall from unbelief, and to hold fast their profession:
"Seeing then that we have a great high priest, that is passed into the heavens, Jesus the Son of God, let us hold fast our profession." Hebrews 4:14
If those fall away in unbelief it is impossible to renew them again to repentance seeing they crucify the Son of God afresh by reverting back to the old priesthood and Levitical system of sacrifice.
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. Hebrews 4:3