As usual, a hit and run post from you with an avalanche of verses without context and understanding. Many of these verses have already been covered, but I will address Hebrews 3:12 for now:
The Lord delivered His people (the Israelites) out of Egypt, but later destroyed those who did not believe (Jude 1:5). God had revealed Himself to these Israelites and chosen them as His people, but does that mean they were ALL saved? The writer of Hebrews is addressing brethren here in verse 12, but does this mean that EVERYONE in this very large group of Hebrews must be a genuine believer? As we read in verse 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 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 only ones in the end who will be identified as those who have become partakers of Christ, will have been those who hold fast the beginning of their confidence steadfast to the end." *These faltering Hebrews who depart from God begin with loud confidence and profession of loyalty. *But later?
The wording of the above verse in Hebrews 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, partakers of Christ, if in the future you hold fast to Christ." Holding fast to Christ is a manifestation of genuine belief and having become partakers of Christ and departing from God is a manifestation of unbelief and failure to have become partakers of Christ. Just as we see in 1 John 2:19, if they had been of us, then they would have continued with us.
In Hebrews 4:2-3, 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. 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.