It was TO Hebrews, discussing how superior what Christ offers is to anything the Jewish religion provided. Its constant theme is the superiority of Jesus.
But if JEWISH Christians can fall away, then GENTILE Christians can. You cannot have it both ways. You cannot claim the warnings of chapter 6 are irrelevant to us because they only apply to Jews - because there is neither Gentile nor Jew in Christ.
And consider Romans 11:
“22 Notice how God is both kind and severe. He is severe toward those who disobeyed, but kind to you if you continue to trust in his kindness. But if you stop trusting, you also will be cut off. 23 And if the people of Israel turn from their unbelief, they will be grafted in again, for God has the power to graft them back into the tree. 24 You, by nature, were a branch cut from a wild olive tree. So if God was willing to do something contrary to nature by grafting you into his cultivated tree, he will be far more eager to graft the original branches back into the tree where they belong.”
Jew or Gentile doesn’t matter. Hebrews was written to Jews, but is not LIMITED to them. Nor do the rules of salvation change for Jews versus Gentiles. If Jews who have come to Jesus can turn back, so can Gentiles!
Jesus answered, “The work of God is this: to believe in the one he has sent.”(John 6:28-29)
“For I have come down from heaven not to do my will but to do the will of him who sent me.
And this is the will of him who sent me, that I shall lose none of all that he has given me, but raise them up at the last day.
For my Father’s will is that everyone who looks to the Son and believes in him shall have eternal life, and I will raise him up at the last day.” (John 6:38-40)
“I give them eternal life, and they shall never perish; no one can snatch them out of my hand.
My Father, who has given them to me, is greater than all; no one can snatch them out of my Father’s hand. I and the Father are one.”(John 10:28-30)
If you confess with your mouth, “Jesus is Lord,” and believe in your heart that God raised him from the dead, you will be saved.
For it is with your heart that you believe and are justified, and it is with your mouth that you confess and are saved. (Romans 10:9-10)
And you also were included in Christ when you heard the word of truth, the gospel of your salvation.
Having believed, you were marked in him with a seal, the promised Holy Spirit, who is a deposit guaranteeing our inheritance until the redemption of those who are God’s possession—to the praise of his glory. (Ephesians 1:13-14)
19 Do you not know that your bodies are temples of the Holy Spirit, who is in you, whom you have received from God?
You are not your own; 20 you were bought at a price. Therefore honor God with your bodies.(1 Cor. 6:19-20)
Now it is God who makes both us and you stand firm in Christ. He anointed us, set his seal of ownership on us, and put his Spirit in our hearts as a deposit, guaranteeing what is to come.(2 Cor. 1:21-22)
All of this happened at our first moment of belief, before we could do anything to either earn or lose our position. The man on the cross beside Jesus is the prototype for this transaction.
Having done something bad enough to get himself executed, he was promised a place in Paradise solely because he believed in his heart that Jesus was the Lord of a coming Kingdom.
Nothing could be clearer, so in study of scripture we use clear verses to help clarify seemingly complicated ones.
I also believe that the letter to Hebrews was from Paul,(possibly transcribed by Luke) he could not sign or claim it because most Jewish believers were afraid of "Saul".