If we go on sinning deliberately after receiving the knowledge of the truth, there no longer remains a sacrifice for sins, but a fearful expectation of judgment, and a fury of fire that will consume the adversaries. Anyone who has set aside the law of Moses dies without mercy on the evidence of two or three witnesses. How much worse punishment, do you think, will be deserved by the one who has trampled underfoot the Son of God, and has profaned the blood of the covenant by which he was sanctified, and has outraged the Spirit of grace? For we know him who said, “Vengeance is mine; I will repay.” And again, “The Lord will judge his people.” It is a fearful thing to fall into the hands of the living God.(Hebrews 10:26-31)
Many countries require a person to have a visa before they are allowed into the country. The visa will often say how long they can stay in the country. If, after entering the country, they sneak back out of the country before the visa is up where there is no entry/exit point and then present themselves to re-enter the country at a proper entry point they would be likely to be in serious trouble.
That is something like what the writer is thinking about here. We received a visa to let us into the Kingdom of God when we first started to follow Jesus. Our writer says we simply cannot leave the kingdom for a while; live in the kingdom of the world; and then return to the kingdom. There is a very important and strong reason why this is so. We may belong to all sorts of clubs: golf, football, book reading or cookery etc., leave the club, cancelling our subscription, and return to it later. But unlike all these clubs the other factor in the kingdom is God himself. On entry to the kingdom we receive the gift of the Holy Spirit. He cannot be accepted one minute and returned months or years later just because we want to do something else.
We effectively do that if we continue in deliberate sin. The Old Testament is very definite about this. There are two sorts of sin: unintended and ‘with a high hand’, that is defiantly, quite deliberately and intentionally (Numbers 15: 27 - 31).. The former can be remedied by offering repentance and sacrifice, but for the second there is no such remedy.
Unfortunately there seems to be a great deal of teaching around in this Western world in which God is a kind of benevolent grandfather figure who will accept almost anything from his grand-children merely patting them on the head and saying ‘don’t do that again - it is not nice’ or some such comment. We must not forget that we are dealing with the Creator and Sustainer of this world of ours and all the universe. He is a holy God who does not like - will not accept - impurity.