No, the rest of Hebrews 6 does NOT teach that the believer who turns away from Christ can ever come back. What you just said is ADDING to the Scripture.
As a matter of fact, Apostle Paul made it clear that our hope in Christ is ONLY for those who 'endure' to the end, and not for those who turn away from Christ...
Heb 6:11-15
11 And we desire that every one of you do shew the same diligence to the full assurance of hope unto the end:
12 That ye be not slothful, but followers of them who through faith and patience inherit the promises.
13 For when God made promise to Abraham, because He could swear by no greater, He sware by Himself,
14 Saying, Surely blessing I will bless thee, and multiplying I will multiply thee.
15 And so, after he had patiently endured, he obtained the promise.
KJV
What you should do is leave... that 'once saved, always saved' doctrine of men you've been taught, and come to the simplicity and reality of God's written Word. The Pre-tribulationalists think God's Word speaks of a pre-trib rapture too, but it does not, yet those who believe that lie are so sure of it.
I didn't say in that post that the believer who turns away from Christ can ever come back. I said that if anyone clings to Christ, they cannot lose their salvation.
But I will say this: if anyone thinks that they have lost their salvation, John 6:37 would tell you that if you come to Christ, casting yourself upon His mercy, He will by no means cast you out.
Psalms 31:22 tells us that thinking you have fallen away and can never come back may be something that you think in haste. But if you make your supplication unto Him, He will hear you.
Job 14:7-11 shows us that we, as trees of righteousness, may even be cut off at the stump; but that at the first scent of water there can be regeneration and renewal.
Then there is the parable of the prodigal son. It is readily apparent that he was dead and lost; and yet he did not cease to be the son of his father; and when he returned he was accepted with open arms.
I could go on and on.
In Acts of the Apostles 27, there is a story where the people in the boat lost all hope of being saved; but only a few verses later they were saved from the power of the storm
In Luke 1:37 and a few other verses, it declares that with the Lord nothing shall be impossible.
In Acts of the Apostles 20:6-12, a boy named Eutychus falls from the third story of the building ans is taken up dead; but Paul raises him from the dead by the power of the Holy Ghost.
And this is representative of the fact that Romans 8 is the third story of the building, Romans 7 is the first story, and unbelief is the first story. A person who falls from the third storyto the second story has not fallen away. But even if someone falls from the third story to the first and dies, scripture testifies to the power of spiritual resurrection by showing forth a physical one.
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As for pre-tribulation rapture, Revelation 3:10 teaches it pretty clearly impaho.