“Therefore, holy brethren, partakers of a heavenly calling, consider Jesus, the Apostle and High Priest of our confession…” (Hebrews 3:1, NASB).
“Take care, brethren, that there not be in any one of you an evil, unbelieving heart that falls away from the living God. But encourage one another day after day, as long as it is still called ‘Today,’ so that none of you will be hardened by the deceitfulness of sin. For we have become partakers of Christ, if we hold fast the beginning of our assurance firm until the end…” (Hebrews 3:12-14, NASB)
The above passages are part of the same chapter. In Hebrews 3:1, Paul calls his audience “holy brethren” and tells them they are partakers of a heavenly calling. My impression is that these people are already saved, in the fold. Yet, he warns these very same people to take care not to let their hearts “fall away from the living God” or be “hardened by the deceitfulness of sin”.
If you doubt that he’s speaking to already saved people, what do you say about the next verse (14) which says, “For we have become partakers of Christ”? Would Paul really tell unsaved people that they had already become partakers of Christ? He then adds a condition to their staying partakers of Christ: “if we hold fast the beginning of our assurance firm until the end…”
Would it make sense to soberly warn people who were already saved to guard their hearts against being hardened and falling away from the living God if it were already impossible for them to be lost? Or, as I am strongly inclined to believe, does the passage indicate that we must choose to remain committed to Him through the exercise of our free will in order to remain in relationship with Him?
If you still have any doubts, read another warning in verse 4:1:
Therefore, let us fear if, while a promise remains of entering His rest, any one of you may seem to have come short of it.
And then, a recommendation (4:11):
Therefore let us be diligent to enter that rest, so that no one will fall, through following the same example of disobedience.
Let us be diligent, therefore, to keep covenant with Jesus, the covenant based upon his sacrifice on the cross.