Hebrews 10:26-30 is known for its shocking and drastic declarations. In the span of five verses, the author deals with both issues of the extent of the atonement and the perseverance of the saints. The author creates a comparative contrast between the Mosaic covenant and the covenant of Christ (28-29). He is claiming that the member of the Mosaic covenant who violated that covenant was judged by the terms of that covenant. He goes on to indicate that the punishment for those who spurn the Son of God will be that much worse. The comparison here involves the similarity between the Mosaic covenant and the Messianic covenant that whoever violates them will be judged according to the covenant of which he is a member. The contrast involves the varying degree of punishment. If condemnation was that bad for the one who violated Moses, how bad do you think it will be for the one who violates the Son of God. Several observations are worth making here.
1. The author of the Hebrews does not presuppose that membership in the covenant of Christ translates into final salvation. He sees this as a commonality between the Mosaic and the Messianic covenants. There are those who can be sanctified by the blood of the covenant of Christ who persist in sin and fail to receive the salvation that is the ultimate and final benefit of the covenant relationship. That is to say, the author of Hebrews did not believe in the final and necessary perseverance of the saints. Note that the verb "to sanctify" (hagiadzo) is from the same Greek root as the word for saint (hagios).
2. Since the author does not presuppose that covenant membership guarantees final salvation, he presupposes that one may be sanctified by the blood of Christ and yet fall away. Thus, he also takes it to be the case that the benefits of Christ's blood extend to some who may ultimately fall from grace and experience the covenantal curse. This means that the author of Hebrews did not believe that the atonement was only intended for those who would be ultimately saved.
So, Hebrews teaches the possibility that the saints may fall away and that the potential benefits of the atonement extend to those who may not be saved. For these reasons, the author of the letter to the Hebrews would not have been a Calvinist.