Hebrews 6:4–6 (MEV): For it is impossible for those who were once enlightened, who have tasted the heavenly gift, who shared in the Holy Spirit, and have tasted the good word of God and the powers of the age to come, if they fall away, to be renewed once more to repentance, since they again crucify to themselves the Son of God and subject Him to public shame.
Do not be deceived, there are those who will tell you that this verse says that a Christian can lose his salvation. It doesn’t say that.
In fact, it says the opposite. A person that once believed cannot fall away. Look at the construction of the sentence.
“those who were once enlightened, who have tasted the heavenly gift, who shared in the Holy Spirit, and have tasted the good word of God and the powers of the age to come”
There is a list of qualifications given; once enlightened, tastes the heavenly gift, shared the Holy Spirit. That last bit is most important. Those who’ve been given the Holy Spirit clearly understand. He doesn’t just go away. Nor does He leave us. Therefore we cannot leave Him.
If one were to fall away, it is impossible to restore them to fellowship. One cannot again crucify Jesus to themselves, that was done once for all. It cannot be done again.
By trying to do something again, one cannot be restored to fellowship. This is speaking specifically to those who were returning to the working of the law, and sacrifices for sin. The writer is saying there is no other sacrifice available for sin. His is in the greater context leading to this statement.
We are being encouraged of a greater Way, One we can wholly rely upon to maintain fellowship.
That is why the Bible calls the Holy Spirit the seal of guarantee of one’s salvation (Ephesians 1:13.) None of us can order Jesus to do something, nor can we order God to do something, therefore we cannot order the Holy Spirit to leave. We cannot break His power that keeps us. Fellowship can’t be restored to a believer, because it really cannot be lost.
What of those who do leave the faith, calling themselves ex-Christians?
That has also been anticipated by God. He even gave us a way to measure the passing of time by them. Those who claim to have been a part of us, having left… Demonstrate they never were really a part of us.
1 John 2:18–19 (MEV): Little children, it is the last hour. As you have heard that the antichrist will come, even now there are many antichrists. By this we know that it is the last hour. They went out from us, but they were not of us, for if they had been of us, they would no doubt have remained with us. But they went out, revealing that none of them were of us.
He even calls those who leave… antichrists.