This is because, to their own harm, they are recrucifying the Son of God and holding him up to contempt. 7 For the ground that drinks the rain that often falls on it and that produces vegetation useful to those for whom it is cultivated receives a blessing from God. 8 But if it produces thorns and thistles, it is worthless and about to be cursed, and at the end will be burned. - Hebrews 6
God's salvation is a gratuitous act depending on His attribute of fulfilled promise bestowed upon the irrevocable committed trust of the human persuaded of his own helplessness whose only hope is in God's Beloved Son and Savior.
However, no principle doctrine is to be founded on a conditional passage such as the one you. This is a letter addressed to Hebrews unto whom the oracles of God's Word was entrusted, which they failed to transmit to the waiting lost Gentile world; and when as a politic body ruled as a theocracy, they rejected the Messiah sent to them. It is impossible to remain as a Christ-rejecting Jew and ever think of escaping Hell and the Lake of fire.
This epistle was not meant to apply to the Gentile world, although it may instruct them of what the Jewish religion was, and why that religion is now done away with, dead, and never to be revived.
Anyone who identifies with it or is a constituent of a religious system derived from the Old Covenant of Law with a sacerdotal priest class is likewise dead to God.