Interesting. I was thinking about this last night and did some reading on this. What you use as proof that we can't fall from God's grace, the Catholic uses as proof that we can. In the footnotes in one of my bibles (NAB) I found this regarding Hb. 6:4-6 ...
4. "... enlightened ant tasted the heavenly gift ..." - This may refer to baptism & the Eucharist, but more probably means our enlightenment by faith and our experience of salvation.
5. "... tasted the good word of God, and the powers of the world to come, ..." - The proclamation of the "word of God" was accompanied by signs of the Spirit's power.
And here's the kicker ... 6. "... recrucifying the Son of God, putting Him to shame." - A colorful description of the malice of apostasy which is portrayed as again crucifying and deriding the Son of God.
Taking this approach, this passage is speaking of those that are in apostasy. They have abandoned God and Jesus.
Thoughts?