Hebrews 10:29

29.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. (185)

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. (186)