"At the round earth's imagined corners" doesn't have a regular title, because it wasn't customary for English poets to give titles to the individual works in a sonnet sequence. The poem is one of nineteen "Holy Sonnets." And, no, Donne is not an arrogant poet who thinks that his poems are sacred objects. The title just means that the poems are about religion. Readers, including us, commonly refer to the poem by its first phrase, or as Holy Sonnet 7, its number in the 1635 edition of Donne's poetry. In the 1633 edition, the sonnet was fourth in the sequence.