The Galatians fell.
"2 Mark my words! I, Paul, tell you that if you let yourselves be circumcised, Christ will be of no value to you at all.
4 You who are trying to be justified by the law have been alienated from Christ; you have fallen away from grace." - Galatians 5:2,4
Agreed. No one has the power in and of themselves to save themselves or keep themselves saved.
We can only believe, and keep believing, by God's grace.
I just cited one above.
And, Hebrews 10:14, 26-31 talks about sanctified people who have been made perfect forever by the one time sacrifice of Jesus who will then be subject to the raging fire judgment of God if they purposely trample on that blood that sanctified them.
Matthew 18:6-9, and Luke 9:42-47 talk about little ones who believe in Jesus stumbling so as to go to hell.
See, Jesus' sacrifice being 'forever' doesn't mean you always have the benefit of his sacrifice
no matter what. It means it never runs out like the old covenant sacrifices did and had to be made over again.
And eternal life doesn't mean that, while in the body, you have that life for eternity
no matter what. It means the life itself that you receive will never run out, like it did under the old covenant.
And because this is true we are exhorted to keep believing in Jesus in order to keep that which our God given faith secures for us apart from the merit of the works of the law.