John Calvin provides some helpful comments on passages like Hebrews 6:4-6, 10:26, and 12:7.

These passages in Hebrews say,

⁴ For it is impossible, in the case of those who have once been enlightened, who have tasted the heavenly gift, and have shared in the Holy Spirit, ⁵ and have tasted the goodness of the word of God and the powers of the age to come, ⁶ and then have fallen away, to restore them again to repentance, since they are crucifying once again the Son of God to their own harm and holding him up to contempt. – Heb 6:4-6

For if we go on sinning deliberately after receiving the knowledge of the truth, there no longer remains a sacrifice for sins. – Heb 10:26

For you know that afterward, when he desired to inherit the blessing, he was rejected, for he found no chance to repent, though he sought it with tears. – Heb 12:17

Calvin, is this saying that God will refuse forgiveness to someone who repents?

To some it seems harsh, and at variance with the divine mercy, utterly to deny forgiveness to any who retake themselves to it. This is easily disposed of. It is not said that pardon will be refused if they turn to the Lord, but it is altogether denied that they can turn to repentance, inasmuch as for their ingratitude they are struck by the just judgment of God with eternal blindness.

What about those like Esau who seem to repent?

There is nothing contrary to this in the application which is afterwards made of the example of Esau, who tried in vain, by crying and tears, to recover his lost birthright; nor in the denunciation of the Prophet, “They cried, and I would not hear.” Such modes of expression do not denote true conversion or calling upon God, but that anxiety with which the wicked, when in calamity, are compelled to see what they before securely disregarded, viz., that nothing can avail but the assistance of the Lord. This, however, they do not so much implore as lament the loss of. Hence all that the Prophet means by crying, and the apostle by tears, is the dreadful torment which stings and excruciates the wicked in despair. It is of consequence carefully to observe this: for otherwise God would be inconsistent with himself when he proclaims through the Prophet, that “If the wicked will turn from all his sins that he has committed,”—“he shall surely live, he shall not die,” (Ezek. 18:21, 22). And (as I have already said) it is certain that the mind of man cannot be changed for the better unless by his preventing grace. The promise as to those who call upon him will never fail; but the names of conversion and prayer are improperly given to that blind torment by which the reprobate are distracted when they see that they must seek God if they would find a remedy for their calamities, and yet shun to approach him.

* Institutes III, iii, 24.