Hebrews 10:26-31 I believe.
Jeff was not quite remembering it correctly
This is good now I'll explain how he is not talking about losing his salvation but living in fear of losing his salvation: (mine is in parentheses)
26 For if we go on sinning deliberately after receiving the knowledge of the truth,
(after receiving the knowledge that you have been saved by grace through Christ's sacrifice you boldly continue to live in your sinful ways)
there no longer remains a sacrifice for sins, but a fearful expectation of judgment, and a fury of fire that will consume the adversaries. (You will no longer remember Christ's sacrifice but will fear his judgement for your sins)
Anyone who has set aside the law of Moses dies without mercy on the evidence of two or three witnesses. (You remember in the old way there was no mercy for sinners)
who has trampled underfoot the Son of God, and has profaned the blood of the covenant by which he was sanctified, and has outraged the Spirit of grace? (Who has done these sinful things? You have and by your guilty conscience you feel you have lost your grace)
“Vengeance is mine; I will repay.” And again, “The Lord will judge his people.” It is a fearful thing to fall into the hands of the living God. (You remember all the things that were said about sinners)
(See, the thing is you can't lose your grace because you never earned it in the first place. You can only torture yourself with guilt by not living up to the grace that was given.)
Romans 2:29
No, a man is a Jew because he is one inwardly, and circumcision is a matter of the heart, by the Spirit, not by the written code. Such a man's praise does not come from men, but from God.
2 Corinthians 12:8-9
8 Three times I pleaded with the Lord to take it away from me.
9 But he said to me, “My grace is sufficient for you, for my power is made perfect in weakness.” Therefore I will boast all the more gladly about my weaknesses, so that Christ’s power may rest on me
Ephesians 2:8-9
8 For it is by grace you have been saved, through faith—and this is not from yourselves, it is the gift of God—
9not by works, so that no one can boast.