For if we sin wilfully after that we have received the knowledge of the truth, there remaineth no more sacrifice for sins, But a certain fearful looking for of judgment and fiery indignation, which shall devour the adversaries. He that despised Moses' law died without mercy under two or three witnesses: Of how much sorer punishment, suppose ye, shall he be thought worthy, who hath trodden under foot the Son of God, and hath counted the blood of the covenant, wherewith he was sanctified, an unholy thing, and hath done despite unto the Spirit of grace? For we know him that hath said, Vengeance belongeth unto me, I will recompense, saith the Lord. And again, The Lord shall judge his people. It is a fearful thing to fall into the hands of the living God." (Hebrews 10:26-31)
Clearly this passage refers to a saved, born-again person, as no unsaved person can ever be said to have been "sanctified" by the blood of Jesus Christ.
"If any man see his brother sin a sin which is not unto death, he shall ask, and he shall give him life for them that sin not unto death. There is a sin unto death: I do not say that he shall pray for it. All unrighteousness is sin: and there is a sin not unto death." (1 John 5:16-17)
Again, this is referring to true born-agian Christians, as here described as, "brother".
Here we have a clear example, where the Greek ἀδελφός, though masculine, is used for both "brother and sister", though there are some who cannot grasp this!
Do these passages speak of Christians who can ultimately fall away from grace, as the language strongly suggests?