Our two souls therefore, which are one, Though I must go, endure not yet
Now we are hot and heavy with Donne's theology. He is practically quoting the Old Testament book of Genesis here, which establishes marriage as making two individuals into one unit.
Like any good metaphysical poet, Donne doesn't shy away from a paradox. He deliberately uses the words two and one in the same line to emphasize the confusing, mysterious force of wedded love.
Line 22 gives us our second big enjambment, or harsh line break. We get the verb, telling us that the two souls will endure something, but we don't know what yet.
A breach, but an expansion, Like gold to airy thinness beat.
Now we figure out what we aren't enduring: "a breach." "Breach" is a harsh word, with its B that explodes out of our mouth and its screeching long E sound. It fits perfectly. More than that, the enjambment itself made us feel a break in the grammar, which mirrors the meaning as well.
Hmm. How can a breach also be an expansion? This is yet another paradox that will have to be explained: they won't break—they will expand… somehow.
The alliteration with rapid B sounds at the beginning of the line also contrasts with the long sound of the word "expansion."
Line 24 is one of Donne's easier analogies, both in form and content. It's a simple simile and only takes one line to spit out, so that's nice. But if you are at all familiar with metal-working (and who isn't, really?), it's also a clear and straightforward image. Gold is a soft metal, easy to hammer and work with. It can be hammered ("beat") into super-fine gold foil, and a little bit can go a long way. Where other metals or materials would break when you stretched or beat them, gold retains its "one-ness," even across a great distance.
The vowels in line 24 are mostly high and melodic, indicating the airy lightness Donne is talking about. (Check out "Sound Check" for more stuff on the sound of this poem.)