Take heed, brethren, lest there be in any of you an evil heart of
unbelief, in departing from the living God (Hebrews 3:12).
Take note of the words:
All the letters in the NT are addressed to brethren, yet that does not mean that everyone in the group of professing believers is a genuine believer. Don't be so naive. Evil heart of unbelief is the opposite of belief and exposes make believers.
Hebrews 3:14 - For we
have become [past tense Gk. verb, gegonamen, meaning we have become already] partakers of Christ, if we hold fast the beginning of our confidence steadfast to the end. Notice that this is essentially a repeat of verse 6, where we have read: but Christ was faithful as a Son over His house -
whose house we are, if we hold fast our confidence and the boast of our hope firm until the end.
The wording is not - "and you will become a partaker of Christ (future indicative) if you (future indicative) hold fast." It is rather - "you have been, and now are a partaker of Christ" (demonstrative evidence) if in the future you hold fast the beginning of your confidence steadfast to the end."
The point is that not all of these Hebrews have become partakers in their promised Messiah and the only ones in the end who will be identified as truly born again Hebrew Christians who have partaken in Messiah, will have been those who have held fast the beginning of their confidence steadfast to the end.
Those faltering Hebrews who depart from God begin with loud confidence and profession of loyalty. But later? Future perseverance is proof of genuine conversion.
Just as in Hebrews 4:1-2, we read - "For indeed the gospel was preached to
US as well as to THEM; BUT the word which THEY heard did not profit THEM, not being mixed with faith in those who heard it. For
WE who have believed do enter that rest, as He has said: "So I swore in My wrath, 'They shall not enter My rest," although the works were finished from the foundation of the world.
Obviously, not all of these Hebrews were believers. Notice that verses 2-3 make a
distinction between US who have BELIEVED and do enter that rest and THEM who heard the word but did not mix faith with what they heard and will not enter that rest because of UNBELIEF.