The warnings in Hebrews 6 scare me almost more than any other passage in the Bible. Huge volumes of ink have been spilt about whether the writer describes:

Non-Christians who have tasted significant blessing and spiritual encounter with Jesus and then decided they want nothing more to do with him; ie people who aren't born again and therefore are not losing salvation that they didn't have to start with
Christians who decide they aren't interested in the things of God any more and who walk away; ie people who were really born again and who therefore lose their salvation

There are some deep matters contained in that distinction about election, assurance and whether God allows real believers to fall away, but I don't think that is the most helpful point to make. Not in a short blog, anyway.

Whichever it is, the verses describe people who have had significant spiritual experience of God:

They have tasted the heavenly gift
They have shared in the Holy Spirit
They have tasted the goodness of the word of God
They have tasted the powers of the coming age

Nevertheless they clearly commit the unforgiveable sin of blasphemy against the Holy Spirit - the only sin in scripture that removes the possibility of repentance. Here expressed in terms of crucifying Christ all over again, which I take to mean agreeing with Jesus' crucifixiers that it was right to kill him and denying the truth of his victory over sin and death.

In other words you can be a significant taster and not be justified by God. Most scarily, we can even fool ourselves, perhaps for long periods of time, but be nothing more than tasters and not be justified by God. That is the danger of spiritual drift and careless assumption.

Is there any way to tell which we are? Yes there is. In fact the writer gives us three tests:

In v7-8 these people are compared with land that drinks in rain but produces worthless thorns and thistles. People who belong to God are like land that drinks in rain and produces a good crop. They are blessed by God. But not these people. They receive but only give out worthlessness. We are meant to examine our lives to see if we are producing the fruit of godliness and righteousness
These people are compared to those the writer addresses in v9-11. The difference is that while his audience are currently apathetic, there is plenty of past evidence of their love for God and their help for the people of God. It is worrying that it is only in the past, and he urges them to renew their love and diligence, but it seems that there was some real fruit previously
He has previously berated them for not wanting to go on to maturity. The implication is that if you aren't interested in going on, then you are sliding back. And the final endpoint of sliding back is that you might fall into this eternally terrible position against which he warns

So, the test for us from this sobering passage is: are we going with God today? Are we producing fruit in keeping with righteousness today? Or are we tasting and enjoying goodies from God but with no real desire to be like God? Are we satisfied with having "made a commitment", ticked the "sins-forgiven-going-to-heaven box" but with no ongoing interest or desire to grow in God? In that case our danger is palpably real and close

We want each of you to show this same diligence to the very end, in order to make your hope sure. We do not want you to become lazy, but to imitate those who through faith and patience inherit what has been promised