The above post gives you great historical information. You might also consider the "literary era" with which John Donne is associated. He is usually characterized as a Metaphysical Poet and is sometimes even credited with developing this style of poetry. The poems are marked by extremely clever and intellectual metaphors called conceits. In this poem, he is comparing he and his love to hemispheres of earth and stating that they should be indivisible.
He is suggesting that they are they perfect balance of the elements, and therefore are a perfect love. He also suggests that their relationship represents the whole world. He is certainly making a lot of grand statements!