The speaker is having none of Psyche's version of things. He's convinced himself that this goddess of love is here to save him, and he's sticking to his story.
He tells Psyche to buck up, and insists that her terror is "nothing but dreaming."
Let us on by this tremulous light! Let us bathe in this crystalline light!
Even though Psyche wants to run away, the speaker wants to do the opposite. He's urging her to keep moving ("let us on") in the flickering, unstable ("tremulous") light of the moon. He's so excited about what he's seeing that he wants to "bathe" in the pure, clear ("crystalline") light.
Its Sybilic splendor is beaming
He realizes Psyche isn't really convinced, so he tries to talk up his plan a little more. He describes the "Sybilic splendor" of the light.
The sibyls were oracles in ancient Greece, they could tell the future. (You might be familiar with the oracle from the Percy Jackson books.) Like Astarte, this is an image of a powerful female force, the kind of woman who could really help you out…if she wanted to.
With Hope and in Beauty to-night:—
The speaker tells his soul that the light is full of "Hope and […] Beauty." That's exactly what this poor guy needs, too, since things have seemed pretty ugly and hopeless so far.
Can you feel him reaching here, clinging to the faintest hope? Poe is really good at writing about mental instability and desperate desire. We think there's a little of both of those things here.
See!—it flickers up the sky through the night!
There is something kind of enchanting and beautiful about the images the speaker is weaving here. When he tells us to "See!" the beautiful light, it really makes us want to play along, to believe in the flickering light like he does.
Ah, we safely may trust to its gleaming, And be sure it will lead us aright—
He assures Psyche that they "safely may trust" the gleaming light. He's confident that it will lead them in the right direction ("aright").
Now, if we were hanging out with this guy, we might like a little bit of proof. Like, how do you know we'll be all right? That's not what he's about though. He's running on pure faith here, on his gut instinct.
We safely may trust to a gleaming That cannot but guide us aright, Since it flickers up to Heaven through the night."
Since he can't really come up with good reasons for going toward the light, the speaker just repeats himself, reassuring Psyche that there's nothing to fear.
He suggests, not so convincingly, that they can trust the light because "it flickers up to Heaven." This is a little echo of lines 45-46, and it reminds us that this guy's fantasy is all about escaping up to heaven, leaving the earth behind.