For follow-up discussion and general commentary on the topic. Comments are sorted chronologically.
The entire Book of Hebrews was written to Hebrews (Jews) who had ...... almost ...... accepted Jesus as Savior. They were not Christians. The author of Hebrews seeks to warn them of the futility of going back to Judaism. Judaism would not save them in the Age of Grace.
Yes going back on the faith of Jesus, Paul addressed this very type of situation in Galatians 2 when Peter came to Antioch. But if you ever fall away from his commandments you can lose your salvation.
Hebrews was written primarily to ethnically Jewish Christians, not to 'almost' believers. They are holy brothers and sisters in Christ, who share in the heavenly calling. They are the house of God under the headship of Christ. They fellowship in Christ. They have believed, obeyed, and professed their faith. [Heb 2:1-4, Heb 2:10-18, Heb 3:1-4, Heb 3:12-14, Heb 4:1-3, Heb 4:14-16, etc.] Yet, Paul continuously admonishes them to hold firmly to their faith and conviction, and not to turn away.
It is those who began in faith, but did not hold firm, that he speaks of in Heb 6:4-They have seen the light (II Cor 4:6), they have ate of the heavenly gift (John 6:33), they have shared in the Holy Spirit (I Cor 12:13, II Peter 1:4, Eph 3:6, Heb 3:14, Col 1:12, Phil 1:7, Eph 4:4, etc), they have tasted the goodness of the word of God (Psalm 38:4) and shared in spiritual gifts (1 Cor 12:4), and have repented (Acts 2:38). Yet, they then fell away (Heb 6:6, Heb 10:29, II Pet 2:21).
Paul holds these people up as a warning, that other Christians might not become inert, but rather inherit the promises through faith and long suffering. (II Pet 1:3-11 is a more detailed example of how to grow in faith vs. stagnate/be unproductive and risk 'forgetting' that one has been cleansed from past sins'.
[Related eBible topic: https://ebible.com/questions/6750-how-do-christians-make-their-calling-and-election-certain-2-peter-1-10]