Technically this letter is addressed to the Hebrews so the term Christians isn't relevant. All are One in Christ
I should address what I do not agree on. This is the one.
There is a difference between a Hebrew who became a Christian, and one who did not. So to say we are all one in Christ when there are some Hebrews that do NOT have Christ is what I do not agree with. Only the Hebrews and Gentiles that became Christians had any way of being sanctified, so the term "Christian" is VERY relevant.
I see I was too hasty. Now what I do agree with:
Christians will openly admit that they still sin because they can't keep God's law through their best efforts.
I understand what you are saying, but calling them Christians is a term they do not deserve. A true Christian is filled with the Holy Spirit, and empowered to keep the laws written on their hearts. They are born again. On this we agree. To become a true Christian those wanting to be Christians must repent of their inadequacy and self efforts, and humbly admit they can do nothing without His help. Ask to be filled with His power because you hate your sin, and want to live a life pleasing to Him. He will give you His Spirit. It is a life-changing experience. Also the word of God becomes alive and you will understand verses you had wrong your whole life. Those who love darkness, but do not want to pay the cost, will turn to the false teachers that only present Jesus as their ticket to heaven that paid their consequences for the sins you they still keep committing.
Christians will openly admit that they still sin because they can't keep God's law through their best efforts. They never seem to be able to see that they are admitting that they are still operating under the parameters of the Old Covenant rather than the New which is according to God's promise, and the operation of his spirit indwelling in the new creation who are created to keep his commandments. They are created to do good works. The bible refers to them as "fruit" because they are produced systemically.
I agree.
Those "Christians" who are not filled with the Spirit of Christ, can only use their own efforts which leaves them far from righteous or on the path to salvation. Only those with His Spirit belong to Him. Romans 8:9. All others that call Him Lord, He will say depart from me, I never knew you, you workers of iniquity. In other words, they still sin. Jesus was manifest to take away our sin, and in Him there is no sin. To those without His Spirit, are still slaves of sin, so repent. Having the Holy Spirit is not just something you are told to believe doctrinally. It is a real experience that you can't deny.