The “falling away” recorded in the Book of Hebrews 5:11-6:6 refers to first century Jews who had been led by the Holy Spirit to leave Judaism and confess Christ, who are now in danger of resisting the Holy Spirit under pressure of persecution, and hardening their hearts against the conviction of the Holy Spirit and go back to the “Elementary” or beginning and incomplete teachings about Christ under the Old Testament system of types and foreshadows.
They are warned in Hebrews 3:7-8: “So as the Holy Spirit says: ‘Today, if you hear His voice, do not harden your hearts”. They were led by the Holy Spirit to see their need of Christ, but after repeated resistance they were in danger of grieving the Spirit away: “If we deliberately keep on sinning (by hardening our hearts) after we have received the knowledge of the truth, (after the Holy Spirit convicts us of our need of Christ) no sacrifice for sin is left (If Christ is rejected, there is no other sacrifice for sin) But only a fearful expectation of judgment and a raging fire that will consume the enemies of God”.
In verse 29 these people are said to have “trampled the Son of God under foot, who has treated as an unholy thing the blood of the covenant that sanctified him, and who has insulted the Spirit of Grace?
To harden our hearts to the Holy Spirit’s conviction of our need of Christ is to walk on the blood of Christ as a common thing, and insulting the Holy Spirit who has brought this conviction to us.
Christians who are truly born again cannot lose their salvation because the Holy Spirit lives in them. But unbelievers who are on the verge of receiving Christ can harden their hearts to the Holy Spirit’s conviction and “fall away” from receiving Christ.