"For though by this time you ought to be teachers, you need someone to teach you again the basic principles of the oracles of God. You need milk, not solid food, for everyone who lives on milk is unskilled in the Word of Righteousness, since he is a child. But solid food is for the mature, for those who have their powers of discernment trained by constant practice to distinguish good from evil.
Therefore let us leave the elementary doctrine of Christ and go on to maturity, not laying again a foundation of repentance from dead works and of faith toward God, and of instruction about washings, the laying on of hands, the resurrection of the dead, and eternal judgment. And this we will do if God permits. For it is impossible to restore again to repentance those who have once been enlightened, who have tasted the heavenly gift, and have shared in the Holy Spirit, and have tasted the goodness of the Word of God and the powers of the age to come, if they then fall away, since they are crucifying once again the Son of God to their own harm and holding him up in contempt."
Instruction about washings: The Old Covenant cleansing laws for purification.
The laying on of hands: When someone would lay their hands on the sacrifice and symbolically pass their sins onto the animal.
He is telling these people who have heard and understood the Gospel, not to go back to the old covenant. These things were just shadows of Who had now come!
In other words, how are you going to convince someone who has heard and understood the Gospel, if they then reject it, to believe it? If you have seen the truth and known the truth, then turn away from it, how can you be brought back?
Sounds familiar doesn't it?
The scribes had once seen Jesus cast out a demon by the finger of God, and then willfully rejected what they had just witnessed. They saw the truth and then rejected it by saying that Jesus cast out demons by the help of demons. Remember what Jesus said afterward?
"Truly, I say to you, all sins and blasphemies will be forgiven the children of man, and whatever blasphemies they utter, but whoever blasphemes the Holy Spirit never has forgiveness, but is guilty of an eternal sin."