"Hebrews is a sermon intended to be read aloud in the Christian assembly
Most New Testament epistles have at least some affinity with the sermonic genre in that they were generally designed to be read out to the Christian congregations. But Hebrews calls itself a 'word of exhortation' (13:22), which was a term used in contemporary Judaism and early Christianity to refer to the sermon in a synagogue or church gathering. It is used in this way in Acts 13:14-15, where Paul and his companions are invited to preach in the synagogue at Pisidian Antioch: 'After the reading from the Law and the Prophets, the rulers of the synagogue sent a message to them, saying, "Brothers, if you have any word of encouragement [or 'exhortation'] for the people, say it."' Quite clearly, Paul (along with his companions) is being invited to deliver the Sabbath-day sermon. In a later Christian context, a fourth-century liturgy designates the sermon 'words of exhortation' (Apostolic Constitutions 8.5).
Added to the designation the writer gives his sermon, the form of the discourse indicates that Hebrews is no ordinary letter. It lacks a traditional epistolary opening, but rather launches straight into the substance of its 'preaching', setting it apart in form from other New Testament letters as more markedly sermonic in genre. A number of features of Hebrews indicate its fundamentally 'oral' rather than written character: the writer's use of the first person plural (which 'enables the speaker to identify with his listeners while addressing them with authority'); frequent references to speaking and hearing, rather than writing (2:5; 6:9; 8:1; 11:32); and the regular alternation between exposition and exhortation. As noted above, Hebrews is arguably the earliest extant full-length Christian sermon. [fn4]" (Griffiths, Jonathan. Preaching in the New Testament: An exegetical and biblical-theological study, NSBT, 104-105.)
[fn4] Vanhoye (1989:3), who has perhaps devoted more attention to the structure and genre of Hebrews than any other recent scholar, concludes that '"The Letter" to the Hebrews is simply not a letter ... it belongs to the genre of preaching. In fact, it is the only example we have in the New Testament of the text of a sermon which has been preserved in its entirety.' Arguably, the designation 'letter' remains appropriate for Hebrews because the category of 'letter' in the NT is probably broad enough to encompass a written sermon like this one (so, rightly, O'Brien 2010: 21). But Vanhoye's conclusion that Hebrews is fundamentally a sermon is surely correct and is now widely accepted. Cockerill's (2012) recent commentary on Hebrews reflects this conclusion; he regularly refers to the author as 'pastor' and to the discourse as the 'sermon' (and sometimes also to the intended recipients as the 'congregation').