The author of Hebrews is writing to all people who profess to be Christians. And since he doesn't know who is born again and who isn't, he's warning
everyone to make sure that we don't turn away from God.
However, in Hebrews 6:9, God tells us; "Even though we speak like this, dear friends, we are confident of
better things in your case-things that accompany salvation." He is clearly separating the true sheep from the false sheep by telling the specific people to whom the letter was written that he's confident they are born again.
So put these passages together along with 1 John 3:8 and 5:18 to see why true born again believers cannot fall away. As Romans 11:18 says, "you do not support the root, but the
root supports you."
And lastly, out of the 4 groups in the parable of the sower, only the last group remains form because the last group has the root (the Holy Spirit). So unfortunately, most people who hear the word will eventually fall away. Hebrews is simply another warning about the cost of apostasy.
