The following passage is essential in needing to be understood before Hebrews 6:4-6 can be understood without a problem.
Acts 21:20-26
20 When they heard this, they praised God. Then they said to Paul:
“You see, brother, how many thousands of Jews have believed, and
all of them are zealous for the law. 21 They have been informed that
you teach all the Jews who live among the Gentiles to turn away from
Moses, telling them not to circumcise their children or live according
to our customs. 22 What shall we do? They will certainly hear that you
have come, 23 so do what we tell you. There are four men with us who
have made a vow. 24 Take these men, join in their purification rites and
pay their expenses, so that they can have their heads shaved.
Then everyone will know there is no truth in these reports about you,
but that you yourself are living in obedience to the law. 25 As for the
Gentile believers, we have written to them our decision that they should
abstain from food sacrificed to idols, from blood, from the meat of
strangled animals and from sexual immorality.”
26 The next day Paul took the men and purified himself along with them.
Then he went to the temple to give notice of the date when the days of
purification would end and the offering would be made for each of them."
The Jewish believers were in essence crucifying Jesus afresh each time they insisted in returning to the Temple to offer sacrifices!
It got so bad they convinced even Paul to offer sacrifice in the Temple. God almost killed Paul (sin onto death) for doing it.
Because, Paul who understood grace and the Cross better than anyone, was without excuse for what he let himself be talked into.
That was the problem being addressed in Hebrews 6:4-6!
“You see, brother, how many thousands of Jews have believed, and
all of them are zealous for the law. (animal sacrifices! crucifying Jesus afresh!)
Those Jews were saved. They had been enlightened.
They had tasted the heavenly gift of salvation! But?
Those Jews, would not give up their zeal for the law. .No repentance.
They were stubborn and remained highly religious.
Those Jewish believers went apostate from real Christianity and refused correction.
It had become impossible to bring them back to their senses to repent and correct their ways....
The author of Hebrews was saying in effect....
"Leave them be! They are too far gone! Its impossible anymore to get them to repent.
But.. As for you? Continue on in maturing in Christ and the blessings to come. "
That in essence was what Hebrews 6:4-6 was addressing.
grace and peace!