From the Douay:
6:4. For it is impossible for those who were once illuminated, have tasted also the heavenly gift and were made partakers of the Holy Ghost,
6:5. Have moreover tasted the good word of God and the powers of the world to come,
6:6. And are fallen away: to be renewed again to penance, crucifying again to themselves the Son of God and making him a mockery.
*It is impossible, etc . . . The meaning is, that it is impossible for such as have fallen after baptism, to be again baptized; and very hard for such as have apostatized from the faith, after having received many graces, to return again to the happy state from which they fell.
When we were saved, there were no earth-shaking changes in our powers, if any. But the earliest Christians--all Jews--received powers, on or shortly after the Day of Pentecost, that we can barely imagine:
"And these signs shall follow them that believe; In my name shall they cast out devils; they shall speak with new tongues; They shall take up serpents; and if they drink any deadly thing, it shall not hurt them; they shall lay hands on the sick, and they shall recover." (Mk 16:17-18 KJV)
That places Hebrews 6 in an entirely different light:
"For it is impossible for those who were once enlightened, and have tasted of the heavenly gift, and were made partakers of the Holy Ghost, And have tasted the good word of God, and the powers of the world to come, If they shall fall away, to renew them again unto repentance; seeing they crucify to themselves the Son of God afresh, and put him to an open shame." (Heb 6:4-6 KJV)
Almost immediately after the Day of Pentecost there was an attempt by traditional Jews to "Judaize" those who had received the new powers. In the Acts, the epistles, and in the letters to the seven churches of Asia, some of those attempts are documented. Apparently some were successful, hence Hebrews 6.
This discussion also helped me understand more clearly another scripture I have long pondered: blasphemy against the Holy Ghost. How can you blaspheme something you have never seen or experienced? (well, other than believing you are God, like some of the early Roman emperors, and a few politicians of late? LOL!)
But the earliest Christians knew the Holy Ghost: they received its special powers; and much was expected of them. However, we who called upon the name of the Lord to be saved have received no special powers; therefore not as much is expected of us. At least, that is the way I interpret this verse:
"For unto whomsoever much is given, of him shall be much required . . ." (Luke 12:48 KJV)
Philip