Impossible (adunaton). Strong word as in Heb 6:4, Heb 6:18. See Rom 8:8 for same idea with aresai (areskō, Gal 1:10).
Must believe (pisteusai dei). Moral necessity to have faith (trust, pisteuō). This is true in business also (banks, for instance).
That he is (hoti estin). The very existence of God is a matter of intelligent faith (Rom 1:19.) So that men are left without excuse.
He is a rewarder (misthapodotēs ginetai). Rather, “becomes a rewarder” (present middle indicative of ginomai, not of eimi). Only N.T. example of misthapodotēs, late and rare double compound (one papyrus example, from misthos (reward) and apodidōmi (to pay back) like misthapodosia (Heb 10:35; Heb 11:26).