Disclaimer: I'm working towards attaining my Ph.D in English particularly focusing on Science Fiction and American Literature. That being said, Zelazny is my favorite author and I have studied the Amber novels for years. So, while the following is essentially speculation, I am also not merely making things up as I go.
Zelazny filled the Amber series with numerous allusions and references to other SF and Fantasy works. The first sentence, as far as I am aware is not specifically referencing anything else, though it is imitative of biblical verse. Luckily, however, Zelazny utilized Norse mythology in numerous locations throughout both Amber and many of his short stories. Here, my best guess is that this section is, and perhaps to a greater degree the whole shadow storm that Corwin rides before, a reference to Ragnarok and Heimdallr. With that in mind I'll proceed to the text.
Just a page or so before the quote in question Corwin states:
I touched the Jewel. ... with red pulses of energy corresponding to my heartbeats.
The lightning upon his breast, in this instance, is actually just the jewel of judgement and, obviously, Corwin is passing through this shadow ahead of the shadow storm.
The next part of this holy scripture, as it were, is where it gets interesting. The response "To the ends of the Earth" and the evocation of "the Horn", I believe are direct references to Bifrost and the Gjallarhorn.
In Norse mythology, Heimdallr is the god that watches over Bifrost, the bridge between heaven and Earth. He is also tasked with sounding Gjallarhorn to both summon the other gods to him for aid as well as signal the beginning of Ragnarok or the end of days. During Ragnarok the Earth is completely submersed in water and, upon surfacing, is cleansed and renewed.
Look to the beginning of chapter six:
I rode fewer than a thousand meters to what had been the south, and everything stopped ... I thought then of the stranger in the cave and his words. He had felt that the world was being blotted out by that storm, that it corresponded to something out of a local apocalyptic legend. Perhaps it had. Perhaps it had been the wave of Chaos of which Brand had spoken, moving this way, passing over, destroying, disrupting. But this end of the valley was untouched. Why should it remain?
Then I recalled my actions on rushing out into the storm. I had used the Jewel, the power of the Pattern within it, to halt the storm over this area.
Here, it becomes more clear that the shadow storm is analogous with the earth being submersed; this Shadow with Midgard. Corwin, recast as an Archangel bearing a talisman (or lightning upon his chest), has the ability to part the waters for a time temporarily preserving a small patch of Earth from the cleansing waters of Ragnarok. Yet, being on earth, Corwin cannot possibly know who or what Heimdallr has summoned with Gjallarhorn.
Though I'm not certain, I don't think the first sentence is from anything in particular but this section is only a small part of a larger motif in which Zelazny incorporates characters into extant myths and stories.