"My Madeline! sweet dreamer! lovely bride! Say, may I be for aye thy vassal blest? Thy beauty's shield, heart-shaped and vermeil dyed? Ah, silver shrine, here will I take my test After so many hours of toil and quest, A famish'd pilgrim,—saved by miracle. Though I have found, I will not rob thy next Saving of thy sweet self; if thou think'st well To trust, fair Madeline, to no rude infidel.
Porphyro wants to reassure her that there won't be any fading or pining, asking her if he can be her metaphorical "vassal," which is a medieval title for a servant or subject. This may remind you of how Porphyro described himself as Madeline's "eremite" in stanza 31—he keeps on describing himself as being subjugated to her.
As if that isn't enough, he then goes on with the metaphors and calls her a "silver shrine" and himself a "famish'd pilgrim." Porphyro, buddy, we get it.
He says something kind of interesting, then. While he's in the throes of his "Oh, Madeline, you're just the greatest most heavenly shrine-like thing ever" spiel, he says that he's "saved by a miracle." What miracle do you think that is?
Within the context of the speech alone it could refer to Madeline's love: by offering to let Porphyro serve and worship her she has "saved" him. On the other hand, within the context of the poem as the whole, you can also take the miracle to be all of the semi-magical hoodoo that's been going down because of St. Agnes' Eve. As you've noticed, Porphyro has had some pretty spectacular luck this evening, and even though he and Angela keep claiming that the St. Agnes' Eve ritual is baloney, there's definitely something in the air tonight. A third option could be that the miracle is that crazy moment of "melting" into Madeline's dream.
He tells Madeline he wants to "rob" Madeline from her "nest" (again with the Madeline-is-a-helpless-bird metaphor), promising her again that he's "no rude infidel." If you have a free moment, just go back through this poem and check out all the times that Porphyro has promised that he's not a bad guy.