**That would be a reference to the OT sprinklings, laverings and pourings. Remember, he is writing to the HEBREWS, probably non-believers, not believing gentiles.**
You need to read carefully intros to some of the chapters. Look at these:
2:1 Therefore WE ought to give the more earnest heed to the things which WE have heard, lest at any time WE should let them slip.
3:1 Wherefore, holy brethern, PARTAKERS of the heavenly calling, consider the Apostle and High Priest of OUR profession, Christ Jesus.
4:1-3 Let US therefore fear, lest, a promise being left US....For WE which have belived do enter into rest..
6:1-3 Therefore LEAVING the principles of the doctrine of Christ, let US go on to perfection; not laying again the FOUNDATION of REPENTANCE from dead works, and of FAITH toward God, of the doctrine of baptisms, and of laying on of hands, and of resurrection of the dead, and of eternal judgement. And this we will do, if God permit.
“And this we will do, if God permit”? Do what, if God permits?....Do you think that the writer is telling them that they are not going to preach and teach those things anymore? God forbid.
**That is why he uses OT scriptures to show how the OT for told of the Messiah.**
The writer is taking advantage of their Hebrew upbringing, to show them the deep understanding they need to have, when reaching out to the lost Hebrews.
Look at the situation when Paul wrote this to the HEBREWS. He was not the Apostle to the Hebrews but he still loved his people.
The city was full of Christian believers! Temple priests and sects of the Pharisees were believers! They had seen the miracles, tasted the good gift of God. Forward with Christ!
Then they balked..
Paul in Hebrews compares them to the Israelites in the wilderness who tasted the grapes, liked what they saw, were ready to go into the Holy Land and take it, then they balked, and God gave them no second chance. God allowed them 40 years to die in the wilderness.
Paul is comparing the Hebrews of his day with those in the desert and his letter is his way of pleading with them to continue forward into God’s Rest. Continue forward! Don’t retreat and don’t start over again! Don’t go over the same old ground again! If the nation balks it will be impossible to renew them to repentance even as those who balked at going into the Holy land had 40 years to die in the desert.
They balked, and 40 years after the death of Jesus, the nation and Temple ceased to exist.
If you read HEBREWS with this in mind it becomes a very understandable letter and those warnings for the nation, of losing and falling away suddenly become understandable and lose the terror for believers today.