On this question Hebrews 10 is quite clear and really needs no additional argument.
For if we go on sinning deliberately after receiving the knowledge of the truth, there no longer remains a sacrifice for sins, but a fearful expectation of judgment, and a fury of fire that will consume the adversaries. Anyone who has set aside the law of Moses dies without mercy on the evidence of two or three witnesses. How much worse punishment, do you think, will be deserved by the one who has spurned the Son of God, and has profaned the blood of the covenant by which he was sanctified, and has outraged the Spirit of grace? For we know him who said, “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.
Those who apostatize from the New Covenant are subject to worst penalties than those who violated the Mosaic Covenant (which, according to Reformed Theology, is actually an administration of the covenant of grace, not the covenant of works–the curses in Deuteronomy 28 are no less problematic in principle than any alleged problems with curses in the Gospel Covenant).
The teacher Meredith Kline used to teach that the New Covenant had both blessings and curses. In doing so he said nothing out of the ordinary because he was in agreement with many every other Reformed teachers. (I know this because his material on sacraments, thought I thought it was extreme in denying their essential graciousness, was still seminal to my own thinking.)
But then for reasons that are known only to him, Kline decided that any covenant with both blessings and curses had to be a “covenant of works.” Even if there wasn’t plenty in mainstream (i.e. non-Klinean) Reformed Covenant Theology to correct Kline, the author of Hebrews would still be enough.
But, to repeat, there is no need to use Hebrews to correct run-of-the-mill Reformed Evangelical Theology. Here’s a quotation defending baptism from the website of Third Millennium Ministries:
As a covenant sign, I came to believe that baptism symbolizes the entire covenant, not just one particular covenant blessing, and not even all covenant blessings alone. Rather, the implication would be that, like circumcision, it symbolizes both covenant blessings and covenant curses.
This almost seems to hold to the unnecessary early (and better) Klinean mistake of think that, because the sacraments imply the possibility of curses, we must not call them blessings. But a wedding is a covenant initiation which involves vows and the implied malediction on either partner being unfaithful. Yet, while every one entering marriage needs to soberly reflect on their obligations, that by no means makes a marriage anything less than an act of blessing and love. It certainly does not make marriage a covenant of works. A spouse is required to be faithful to her partner, but she does not earn the love in the relationship by doing so.
By the way, one can actually see how the Klinean concept of covenant affects marriage by looking at the Protestant Reformed Churches in America in which marriage is permanent no matter how much one partner violates the covenant. The offended party is never permitted to divorce and remarry. That is the only way that the graciousness of marriage can be upheld in this way of thinking. Otherwise, marriage becomes a “covenant of works.”
But even if I would tweak the point about baptism quoted above (maybe), it still shows that there is no problem affirming curses of the New Covenant. Likewise, John Frame writes:
God’s covenants are two-edged. Those who are faithful to the covenant receive blessings; those who are not faithful receive curse. Many in Israel falsely trusted in their covenant membership, as if being children of Yahweh they could sin with impunity. But God responded to them with devastation and exile, preserving the faithful remnant. In time it becomes evident that only Jesus is the perfectly faithful remnant. He bears the curse for his people — for all who are joined to him by God’s election (Gal. 3:13; Eph. 1:4). Yet even under the new covenant there are those who attach themselves to God’s church who later prove to be devoid of true faith and outside of God’s electing love. Those receive exceptionally severe curses as those who rebelled against Christ in the face of intimate knowledge (Heb. 6:4-6; 10:26-31). Biblical writers never tire of presenting the enormous consequences of faith or unbelief: the rewards coming to God’s people, the dreadful judgments upon the wicked.
Summoning examples from the Reformed mainstream to substantiate an obvious Scriptural point seems almost blasphemous. The Bible teaches that there are covenant curses in the New Covenant that are distinctive from, because much worse than, the curses of the Mosaic Covenant. If the Reformed Tradition had taken the same path as the Grace Evangelical Society and propounded antinomianism, then that would be so much the worse for the Reformed Tradition. The Bible is completely clear on this issue.
Naturally, the elect never experience such New Covenant sanctions. Rather, they are given a persevering faith that avoids them. In fact, the New Covenant sanctions are means of their perseverance because, as the Westminster Confession states, saving faith reacts with fear of the consequences of unbelief as well as pursuing the blessings associated with faith. Among other things, justifying faith
believeth to be true whatsoever is revealed in the Word, for the authority of God himself speaking therein; and acteth differently upon that which each particular passage thereof containeth; yielding obedience to the commands, trembling at the threatenings, and embracing the promises of God for this life, and that which is to come (ch 14, para 2).