In the sonnet's two quatrains, Wordsworth is saying that we have become so preoccupied with everyday concerns that we are "out of tune" with nature. The sestet suggests a different relationship with the world.
"Suckled" literally means nursed, like from a mother's breast, but here used figuratively.
"Creed" here means belief or faith.
"Outworn" (the past participle of "outwear") means out of date or antiquated. This does not imply that this faith was exterminated by violent means such as crusades. In fact, it implies that people still know this faith once existed, only that that faith's god or gods are no longer worshipped.
"Would rather" is a phrase expressing that "one state of affairs is preferable to another" (Carter and Mccarthy, page 669).
In other words, Wordsworth is saying "he'd rather by a pagan raised in an outdated faith".
A "lea" is an open field or meadow.
"Forlorn" has several meanings, including "lonely" and "miserable". Both meanings are probably at work at the same time.
Rewording Wordsworth's lines in a somewhat less poetic way, we could say,
(…) I would rather be
a pagan raised in an outdated faith
so that I could, [while] standing in a pleasant meadow,
have brief looks / faint ideas that would make me feel less miserable.
Proteus and Triton in the last tercet are Ancient Greek sea gods.
Wordsworth seems to be saying that if he had been raised in the Ancient Greek faith (and sucked it in like mother's milk, an expressly natural image), he wouldn't be so out of tune with nature. However, the phrases "I'd rather be" and "so might I" imply that this is something he cannot or does not realise (see the grammatical concept of irrealis mood. He is stuck in his current faith, which is illustrated by his invocation "Great God". This is presumably the God of Christianity, which does not even tolerate belief in other gods, which the Bible condemns as false gods.
Sources (Excluding Wiktionary)
- Carter, Ronald; McCarthy, Michael: Cambridge Grammar of English: A Comprehensive Guide. Cambridge University Press, 2006.
P.S.: Contrary to what one of the other answers says, the wish for a faith that would (in the poet's mind) bring people closer to nature is not necessarily a choice for ignorance. In July 1827, William Hazlitt quoted the sonnet's last six lines in his essay On Reading New Books. In this essay, he criticises the idea that culture had been dormant or underdeveloped before the French Revolution:
About the time of the French Revolution, it was agreed that the world had hitherto been in its dotage or its infancy; and that Mr. Godwin, Condorcet, and others were to begin a new race of men—a new epoch in society. Every thing up to that period was to be set aside as puerile or barbarous; or, if there were any traces of thought and manliness now and then discoverable, they were to be regarded with wonder as prodigies—as irregular and fitful starts in that long sleep of reason and night of philosophy. (…) There is nothing I hate more than I do this exclusive, upstart spirit.
This is followed by the lines from Wordsworth's sonnet. Hazlitt then also adds, "By despising all that has preceded us, we teach others to despise ourselves." Hazlitt is talking about something completely different than being in touch with nature. However, the reason why I mention this essay is the stance that Ancient Greek culture can't simply be equated with ignorance.