Get out the microscope, because we’re going through this poem line-by-line.
It moves us not. – Great God! I'd rather be
A Pagan suckled in a creed outworn;
- In some sonnets, including this one, important things happen in the ninth line; there is a shift or "turn" that moves the poem in another direction.
- While the speaker reiterates the claim he's been making all along – humanity and nature are alienated from one another – he also tells us how he wishes things were, at least for him, personally.
- He appeals to the Christian God (the capitalization means he has a specific, monotheistic deity in mind) and says he'd rather be a pagan who was raised believing in some antiquated ("outworn"), primitive religion ("creed").
- To wish to be a pagan in 1807 – when the poem was published – would be like saying, "I wish I could wear clothes or do things that were in fashion a thousand years ago."
- Wait a second, he'd rather be a pagan than what? Than someone who isn't moved by nature? Seems like it.
- "Suckled" just means "nursed at a breast" or "nourished."
So might I, standing on this pleasant lea,
Have glimpses that would make me less forlorn;
- The speaker explains why he would rather be a pagan. If he were, then he could look at the land in front of him and see something that wouldn't make him feel so lonely and sad ("forlorn").
- A "lea" is a meadow or open-grassland. Wait a second, wasn't the speaker just telling us about "this sea"? How did we get to the meadow? Maybe he's standing in a meadow overlooking the sea.
- The speaker wants "glimpses" of something, but we don't know what; he suggests that if he were a pagan he would only see things in snatches, for a brief moment, in the blink of an eye.
- And this isn't even guaranteed; he says he "might" have "glimpses."
Have sight of Proteus rising from the sea;
Or hear old Triton blow his wreathed horn.
- The speaker elaborates on those potential "glimpses." He says he might see Proteus coming out of the ocean or Triton blowing his horn.
- Proteus is a sea god in Greek mythology. He had the ability to prophesy the future, but didn't like doing it. If someone grabbed a hold of him and tried to make him predict the future, he would change his shape and try to get away. The modern word "protean" – meaning variable or changing a lot – comes from his name.
- Triton was a son of Poseidon, the Greek god of the sea. He had a conch shell that he blew into in order to excite or calm the waves.
- "Wreathed" means something like twisted, sinewy, having coils; the "wreathed horn" is a reference to Triton's conch shell.