Who is Rintrah?
Rintrah is a figure like John the Baptist from the Old Testament or Elijah the Prophet. Rintrah is meant to be an angry prophet, one who is "crying out in the wilderness" about the deplorable state of the world. In this sense, Rintrah may also express God's anger, and this could signal a change in the world. One might also consider Rintrah as Blake's alter-ego, since he is also crying out for a change in thinking.
Here, Rintrah is a figurative prophet of Blake's time (late 18th century). This is a time of political revolution and uprisings. This is namely due to the French Revolution. Blake was critical of institutional religion and any ideological forces which suppress the freedom and liberty of the individual. The French Revolution gave him, and others, much hope for such a liberation. (Though, they would all be disappointed with the final results.)
Therefore, consider Rintrah as a prophet of disillusionment with what is (or what was) combined with the presaging of what might become of the world in the future. Blake is imitating Biblical prophecies in this poem but he makes use them in order to comment on the supposed progress of his own time in history. Blake is trying to promote a change in thinking, and this stems from his criticism of the rules and hierarchies of institutionalized religion and state power. In particular reference to Christianity, Blake believes strict oppositions are mistakenly thought of as absolutely antithetical to each other: Good/evil, Soul/body Reason/energy, Heaven/hell. Instead, he suggests that these contraries need each other. They are interdependent. If Rintrah's roar is a prophecy of things to come, Blake would like one of those things to be the marriage of contraries: even of the notions of Heaven and Hell.