Amen! Hebrews 3:8-10 says, do not harden your hearts as in the rebellion, in the day of trial in the wilderness, where your fathers tested Me, tried Me, and saw My works forty years. Therefore, I was angry with that generation, and said, 'They always go astray in their heart, and they have not known My ways.' Not descriptive of genuine believers. There is no loss of salvation here. Only a failure to receive it.You were asking about Hebrews 3:12?
Hebrews 3:9-19 KJV
9) When your fathers tempted me, proved me, and saw my works forty years.
10) Wherefore I was grieved with that generation, and said, They do alway err in their heart; and they have not known my ways.
11) So I sware in my wrath, They shall not enter into my rest.)
12) Take heed, brethren, lest there be in any of you an evil heart of unbelief, in departing from the living God.
13) But exhort one another daily, while it is called To day; lest any of you be hardened through the deceitfulness of sin.
14) For we are made partakers of Christ, if we hold the beginning of our confidence stedfast unto the end;
15) While it is said, To day if ye will hear his voice, harden not your hearts, as in the provocation.
16) For some, when they had heard, did provoke: howbeit not all that came out of Egypt by Moses.
17) But with whom was he grieved forty years? was it not with them that had sinned, whose carcases fell in the wilderness?
18) And to whom sware he that they should not enter into his rest, but to them that believed not?
19) So we see that they could not enter in because of unbelief.
They were being called to enter God's rest, but they chose not to from unbelief, instead departing from God, because of their evil heart of unbelief.
An evil heart of unbelief does not describe a regenerate child of God...
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. Considered the truth for a time, then hardened heart and departing from God became their final answer.
*Jude 1:5 - Though you already know all this, I want to remind you that the Lord at one time delivered his people (the Israelites) out of Egypt, but later destroyed those who did not believe. NOT later destroyed those who stopped believing but DID NOT BELIEVE.
Hebrews 3:14 - For we have become 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, in which we 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.
The wording is not - "and you will become partakers 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." Holding fast is proof of genuine conversion.
The point is that not all of these Hebrews have become partakers in Christ and the only ones in the end who will be identified as truly born-again Hebrews who have partaken in Christ, will have been those who have held fast the beginning of their confidence steadfast to the end. Now what about those faltering Hebrews who depart from God, yet begin with loud confidence and profession of loyalty? But then later? Holding fast is proof of genuine conversion.