In 13:22, the writer describes his own letter as a "word of exhortation."
Παρακαλῶ δὲ ὑμᾶς, ἀδελφοί, ἀνέχεσθε τοῦ λόγου τῆς παρακλήσεως, καὶ γὰρ διὰ βραχέων ἐπέστειλα ὑμῖν.
Many have noted sermonic elements in Hebrews. This rather stands out in Hebrews 3:16-18, where the writer asks his readers three very pointed questions, that serve to bring home the portion of Scripture he'd just been using as a warning:
16 τίνες γὰρ ἀκούσαντες παρεπίκραναν; ἀλλ᾽ οὐ πάντες οἱ ἐξελθόντες ἐξ Αἰγύπτου διὰ Μωϋσέως;Thus he stirs up his hearers' attention and involves them in the text. Preachers should never preach for their own amusement, nor simply to discuss a topic. The Word should always have impact; we should always preach before God, to our audience.
17 τίσιν δὲ προσώχθισεν τεσσεράκοντα ἔτη; οὐχὶ τοῖς ἁμαρτήσασιν, ὧν τὰ κῶλα ἔπεσεν ἐν τῇ ἐρήμῳ;
18 τίσιν δὲ ὤμοσεν μὴ εἰσελεύσεσθαι εἰς τὴν κατάπαυσιν αὐτοῦ εἰ μὴ τοῖς ἀπειθήσασιν;