Lines 9-14 Summary

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.