Calvin:
1) The answer to all this is in no way difficult. Princes are called by this name on account of a particular circumstance; as to Israel, the common grace of election is thus denoted; angels are called the sons of God as having a certain resemblance to him, because they are celestial spirits and possess some portion of divinity in their blessed immortality. But when David without any addition calls himself as the type of Christ the Son of God, he denotes something peculiar and more excellent than the honor given to angels or to princes, or even to all Israel. Calvin, Commentary, Hebrews 1:5.
2) That God indeed favors none but the elect alone with the Spirit of regeneration, and that by this they are distinguished from the reprobate; for they are renewed after his image and receive the earnest of the Spirit in hope of the future inheritance, and by the same Spirit the Gospel is sealed in their hearts. But I cannot admit that all this is any reason why he should not grant the reprobate also some taste of his grace, why he should not irradiate their minds with some sparks of his light, why he should not give them some perception of his goodness, and in some sort engrave his word on their hearts. Otherwise, where would be the temporal faith mentioned by Mark 4:17? There is therefore some knowledge even in the reprobate, which afterwards vanishes away, either because it did not strike roots sufficiently deep, or because it withers, being choked up. Calvin, Commentary, Hebrews 6:5.
3) Who has trodden under foot the Son of God, etc. There is this likeness between apostates under the Law and under the Gospel, that both perish without mercy; but the kind of death is different; for the Apostle denounces on the despisers of Christ not only the deaths of the body, but eternal perdition. And therefore he says that a sorer punishment awaits them. And he designates the desertion of Christianity by three things; for he says that thus the Son of God is trodden under foot, that his blood is counted an unholy thing, and that despite is done to the Spirit of grace. Now, it is a more heinous thing to tread under foot than to despise or reject; and the dignity of Christ is far different from that of Moses; and further, he does not simply set the Gospel in opposition to the Law, but the person of Christ and of the Holy Spirit to the person of Moses. The blood of the covenant, etc. He enhances ingratitude by a comparison with the benefits. It is the greatest indignity to count the blood of Christ unholy, by which our holiness is effected; this is done by those who depart from the faith. For our faith looks not on the naked doctrine, but on the blood by which our salvation has been ratified. He calls it the blood of the covenant, because then only were the promises made sure to us when this pledge was added. But he points out the manner of this confirmation by saying that we are sanctified; for the blood shed would avail us nothing, except we were sprinkled with it by the Holy Spirit; and hence come our expiation and sanctification. The apostle at the same time alludes to the ancient rite of sprinkling, which availed not to real sanctification, but was only its shadow or image. The Spirit of grace. He calls it the Spirit of grace from the effects produced; for it is by the Spirit and through his influence that we receive the grace offered to us in Christ. For he it is who enlightens our minds by faith, who seals the adoption of God on our hearts, who regenerates us unto newness of life, who grafts us into the body of Christ, that he may live in us and we in him. He is therefore rightly called the Spirit of grace, by whom Christ becomes ours with all his blessings. But to do despite to him, or to treat him with scorn, by whom we are endowed with so many benefits, is an impiety extremely wicked. Hence learn that all who willfully render useless his grace, by which they had been favored, act disdainfully towards the Spirit of God. It is therefore no wonder that God so severely visits blasphemies of this kind; it is no wonder that he shows himself inexorable towards those who tread under foot Christ the Mediator, who alone reconciles us to himself; it is no wonder that he closes up the way of salvation against those who spurn the Holy Spirit, the only true guide. Calvin, Commentary, Hebrews 10:29.
4) To sum up: We see among all mankind that reason is proper to our nature; it distinguishes us from brute beasts, just as they by possessing feeling differ from inanimate things. Now, because some are born fools or stupid, that defect does not obscure the general grace of God. Rather, we are warned by that spectacle that we ought to ascribe what is left in us to God’s kindness. For if he had not spared us, our fall would have entailed the destruction of our whole nature. Some men excel in keenness; others are superior in judgment; still others have a readier wit to learn this or that art. In this variety God commends his grace to us, lest anyone should claim as his own what flowed from the sheer bounty of God. For why is one person more excellent than another? Is it not to display in common nature God’s special grace which, in passing many by, declares itself bound to none? Besides this, God inspires special activities, in accordance with each man’s calling. Many examples of this occur in The Book of Judges, where it is said that “the Spirit of the Lord took possession” of those men whom he had called to rule the people [Judges 6:34]. In short, in every extraordinary event there is some particular impulsion. For this reason, Saul was followed by the brave men “whose hearts God had touched” [1 Samuel 10:26]. And when Saul’s consecration as king was foretold, Samuel said: “Then the Spirit of the Lord will come mightily upon you, and you shall be another man” [1 Samuel 10:6]. And this was extended to the whole course of government, as is said afterward of David: “The Spirit of the Lord came upon him from that day forward” [1 Samuel 16:13]. The same thing is taught elsewhere with respect to particular actions. Even in Homer, men are said to excel in natural ability not only as Jupiter has bestowed it upon each, but “as he leads them day by day.” And surely experience shows that, when those who were once especially ingenious and skilled are struck dumb, men’s minds are in God’s hand and under his will, so that he rules them at every moment. For this reason it is said: “He takes understanding away from the prudent [cf. Job 12:20] and makes them wander in trackless wastes” [Job 12:24; cf. Psalm 207:40]. Still, we see in this diversity some remaining traces of the image of God, which distinguish the entire human race from the other creatures. Calvin, Institutes 2.2.17.
5) Nevertheless the problem has not yet been resolved. For either we must make Camillus equal to Catiline, or we shall have in Camillus an example proving that nature, if carefully cultivated, is not utterly devoid of goodness. Indeed, I admit that the endowments resplendent in Camillus were gifts of God and seem rightly commendable if judged in themselves. But how will these serve as proofs of natural goodness in him? Must we not hark back to his mind and reason thus: if a natural man excelled in such moral integrity, undoubtedly human nature did not lack the ability to cultivate virtue? Yet what if the mind had been wicked and crooked, and had followed anything but uprightness? And there is no doubt that it was such, if you grant that Camillus was a natural man. What power for good will you attribute to human nature in this respect, if in the loftiest appearance of integrity, it is always found to be impelled toward corruption? Therefore as you will not commend a man for virtue when his vices impress you under the appearance of virtues, so you will not attribute to the human will the capability of seeking after the right so long as the will remains set in its own perversity. Here, however, is the surest and easiest solution to this question: these are not common gifts of nature, but special graces of God, which he bestows variously and in a certain measure upon men otherwise wicked. For this reason, we are not afraid, in common parlance, to call this man wellborn, that one depraved in nature. Yet we do not hesitate to include both under the universal condition of human depravity; but we point out what special grace the Lord has bestowed upon the one, while not deigning to bestow it upon the other. When he wished to put Saul over the kingdom he “formed him as a new man” [1 Samuel 10:6 p.]. This is the reason why Plato, alluding to the Homeric legend, says that kings’ sons are born with some distinguishing mark. For God, in providing for the human race, often endows with a heroic nature those destined to command. From this workshop have come forth the qualities of great leaders celebrated in histories. Private individuals are to be judged in the same way. But because, however excellent anyone has been, his own ambition always pushes him on—a blemish with which all virtues are so sullied that before God they lose all favor—anything in profane men that appears praiseworthy must be considered worthless. Besides, where there is no zeal to glorify God, the chief part of uprightness is absent; a zeal of which all those whom he has not regenerated by his Spirit are devoid. There is good reason for the statement in Isaiah, that “the spirit of the fear of God rests” upon Christ [Isaiah 11:2 p.]. By this we are taught that all estranged from Christ lack “the fear of God,” which “is the beginning of wisdom” [Psalm 111:10 p.]. As for the virtues that deceive us with their vain show, they shall have their praise in the political assembly and in common renown among men; but before the heavenly judgment seat they shall be of no value to acquire righteousness. Calvin, Institutes, 2.3.4.