Critically analysing the themes of Matthew Arnold's "To Marguerite -- Continued" involves investigating the intersections of his views of religion, love, and poetry. The first theme you might consider is that of unrequited love, and in particular, how for Arnold, this seems to have some sort of cosmological and religious significance, which causes it not to be merely a portrait of the poetry of "suffering with no release in action" which he condemned in his 1853 Preface. Next, you should examine how the isolation of humans, like the division of islands by the sea, reflects the atomistic nature of our fallen condition:
A God, a God their severance rul’d!
And bade betwixt their shores to be
The unplumb’d, salt, estranging sea.