I think we could fairly say that the majority of English-speaking Hebrews' scholars believe Hebrews was written to a primarily Jewish audience. I don't think so, but I wanted to quote some of the reasons that have been given in support of a Jewish audience. I comment after each quote.
1. Although he is commenting on the dating of Hebrews, Richard Longenecker (Biblical Exegesis in the Apostolic Period, 144-45) assumes a Jewish audience in the following comment. He has concluded a Jewish audience drawing on William Manson, Yigael Yadin, and especially on Alexander Nairne. This comment is of a piece with this assessment:
"[O]nly on the supposition that the sacrificial worship of the Jerusalem temple still existed as the heart of the nation's life and an intact Judaism continued to offer a live option for the author's readers does the letter become historically intelligible."
The assumption is that the rhetoric of Hebrews is aimed to dissuade a Jewish audience from relying on the Jerusalem cultus. This is a seriously unexamined assumption, however, for Hebrews does not tell its audience not to rely on the Jerusalem cultus (yes, I am aware of 13:9-10). It tells them to rely on Christ's atonement because it works in contrast to the Jerusalem cultus. But such a message might apply as well to console an audience that has lost a temple it relied on as it would apply to an audience tempted to turn to it.
The second unexamined assumption is thus that the audience as Christians did not rely on the Jerusalem temple for atonement in some way. But it remains to be argued that all early Christians understood Christ's atonement to replace the temple cultus in toto. It seems rather more likely to me that many early Christians saw Christ's "sacrifice" as a supplement rather than full replacement. This is exactly the reason why Hebrews is written--to show that Christ's atonement is in fact a full atonement rather than an additional one.
As it plays into the ethnicity of the audience, a third unexamined assumption is that this entire discussion, taken either in the polemic or consolation direction, would obviously apply only to Christian Jews rather than to Christian Gentiles. In other words, it assumes Gentile="liberal in relation to Judaism" while Jew="conservative in relation to Judaism." But this is almost certainly false. In reality there were likely Jewish believers who were far more liberal than many Gentile believers, while there was likely a significant portion of Gentile believers who were more "conservative" in relation to Judaism than Paul.
In short, Longenecker's argument reflects exactly the sort of paradigmatic blind spots that recent scholarship has been working hard to overcome.
I might also mention here a comment James Dunn made in Partings of the Ways some time ago that probably makes the common assumption that Hebrews is a polemic against the Levitical system rather than a consolation in its absence:
"likely we have to envisage a fairly homogeneous Christian-Jewish community somewhere in the diaspora, untouched by the sort of questions Paul raised; but hankering after the ritual and tangibility of the Temple cult, such as the primitive Jerusalem church had enjoyed. In other words, quite likely a group of Christians who had migrated from Jerusalem or Judaea, during or as a result of the Jewish revolt (66-70) and of the Roman suppression of it" (87).
Accordingly, "it is difficult to avoid talking of a parting of the ways in the case of Hebrews" (91).
2. F. F. Bruce (The Epistle to the Hebrews, xxiv-xxx).
Bruce's argument is somewhat nuanced:
a. First, Bruce recognizes a popular argument that is sometimes made for a Jewish audience and rightly dismisses it. In this we affirm Bruce while noting the "unexamined assumption" he thus brings up himself:
"The whole argument [of Hebrews] is conducted against a background of Old Testament allusion; considerable familiarity with the Levitical ritual, and interest in it, are presupposed. Yet all this in itself does not require either the author or the people addressed to be Jewish; we have known at the present time Gentile Christians who were thoroughly familiar with the Old Testament, accepted it as sacred and authoritative scripture, and manifested a lively interest in the details of the Mosaic tabernacle and the Levitical offerings..." (xxv).
Bruce himself, although he favors a Jewish audience, rightly recognizes that this argument is not definitive in any way. I am a Gentile, for example. David deSilva puts it another way: "the Gentile Christian was socialized to view himself or herself as the heir to the titles and promises that belonged to God's chosen people" (Perseverance in Gratitude, 3).
b. Bruce goes on, however, to argue for a Jewish audience:
"[T]he whole 'foundation' of Ch. 6:1ff implies the Jewish antecedents of the readers..." (xxvi)
Let us look at 6:1-2 and see if this comment holds water:
"Therefore, having left the word of the beginning of the Christ, let us bring on perfection, not laying again the foundation of repentance from dead works and faith upon God, the teaching of baptisms and laying on hands, both [the teaching] of resurrection of the dead and of eternal judgment."
I am frankly flabbergasted at Bruce's suggestion that these words imply a Jewish audience, for the author implies that these items--repentance, faith, baptisms, laying on hands, resurrection of the dead, eternal life--are things that coincided with the "word of the beginning of the Christ." They also presumably correlate with "the elements of the beginning of the oracles of God" mentioned in 5:12. In other words, these are elements of foundation laid for the audience when they began to believe the word about Christ.
But most of these would not likely have been items of foundation laid for a Jew coming to believe on Christ! A Jew would quite likely already believe in repentance in relation to sins ("dead works"), faith in God, resurrecton, and eternal life. In short, rather than point to a Jewish audience, these verses provide the strongest evidence for a primarily Gentile audience!
David deSilva argues for a mixed audience, but he supports some Gentile overtones here in the phrase, "repentance from dead works": "More attractive is the suggestion that 'dead works' refer to idols" (216). Again, it is very difficult to see even a Jewish believer referring to performance of the Jewish law as "dead works." We find a similar blind spot in William Lane's commentary (Hebrews 1-8, 140): "the 'dead works' [here as in 9:14] are defined as the external regulations associated with the Levitical priesthood in the earthly sanctuary."
Yet deSilva argues against a purely Gentile implication for these verses when he says things like, "Jewish converts would also need to reorient their own trust in God to incorporate the role of Jesus in securing God's promises and favor" (217). I think it bears tremendous argument that Christian Jewish saw their faith in God as substantially different from their faith in God before they believed on Christ. The difference is their faith in what God has done rather than their faith in God per se. I can't think of any NT evidence that the phrase "faith in God" took on such substantially different connotations for believers that it would have been obvious that the phrase meant something significantly different for a believing Jew than it did for a non-believing Jew.
Harold Attridge (The Epistle to the Hebrews) notes rightly, "It is striking how little in this summary is distinctive of Christianity" (163). I have failed, however, to find a clear cut statement in his commentary on what he thinks of the ethnicity of the audience is. He presents the arguments for a Gentile audience second and is clearly has no problem with the idea of Gentiles in the audience. My guess is he would go with mixed.
c. Bruce's comment goes on: "... implies the Jewish antecedents of the readers, as does also the description of Christ's death in Ch. 9:15 as procuring 'redemption' of the transgressions that were under the first covenant" (xxvi).
Later on the same page he says that the author's "insistance that the old covenant has been antiquated is expressed with a moral earnestness and driven home repeatedly in a manner which would be pointless if his readers were not specially disposed to live under that covenant."
The false assumption here, however, is of a piece with the very assumption Bruce himself has already dismissed, namely, that Gentile believers would not have adopted a perspective in which they believed the Jerusalem cultus to provide atonement for their sins and that they would not have come to associate themselves with God's covenant with Israel even though they had not submitted to circumcision.
The blind spot is the assumption that the early Christians already had the perspective of Hebrews all along, namely, that Christ's death provided the only necessary atonement for sins, that after his death there was no longer any need for sacrifice at all. But this is not at all likely! They would likely have conceived of Jesus' death more along the lines of the Maccabean martyrs, through whom God brought His wrath on Israel to an end. Such an "atoning sacrifice" certainly would not imply to them the end of any need for sacrifice at all! Indeed, we fail to appreciate the potency of Hebrews when we do not realize how revolutionary its suggestion that Christ is the end of all sacrifice is!
My purpose here has been to show not only how weak most arguments for a Jewish audience for Hebrews are, but even moreso to point out paradigmatic blindspots in false assumptions about Judaism and Jewish Christianity that play into them. Of course I could now turn to positive arguments for a Gentile audience, but that will have to wait for another day...