and we both know that Hebrews 3 is about Israel, who never really believed after being freed from Egypt, so that is not a viable go-to verse.
Amen! Hebrews 3:8 - Do not harden your hearts as in the rebellion, In the day of trial in the wilderness, 9 Where your fathers tested Me, tried Me, And saw My works forty years. 10 Therefore I was angry with that generation, And said,
'They always go astray in their heart, And they have not known My ways.'
Verses 18-19 - And to whom did He swear that they
would not enter His rest, but to those who did not obey? So we see that they
could not enter in because of unbelief. That explains the hardened heart. It took them in the opposite direction of God. Heard the truth for a time, but then hardened heart and departing from God became their final answer. Jude 1:5 - The Lord delivered His people (the Israelites) out of Egypt,
but later destroyed those who did not believe.
*Compare with Hebrews 4:1 - Therefore, since a promise remains of
entering His rest, let us fear lest any of you seem to have
come short of it. 2 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. 3 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 genuine believers. *Notice that verses 2-3 makes 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.
Hebrews 3:14 - For we
have become [past tense Gk. verb, gegonamen, meaning we have become already] partakers of Christ,
(demonstrative evidence) 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, (demonstrative evidence) if we hold fast our confidence and the boast of our hope firm until the end. Notice the wording is not - "and you will become partakers of Christ (future indicative) if you (future indicative) hold fast to the end." It is rather - "you have been, and now are, partakers of Christ if in the future you hold fast."
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 Hebrews who have been partakers of Christ 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. 
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