Milton is the master of the stately sounding poem, and "Methought I Saw my Late Espoused Saint" is no exception. Not only does he use grand concepts drawn from Greek mythology, the Old Testament, and Christian theology to describe his "late espoused Saint," he also uses the meter of the poem to reinforce important moments.
Take line 3, for example: "Whom Jove's great son to her glad husband gave." Almost every syllable of this line has an accent over it (a stress). So many stresses give this line a heavy, ponderous feel. Many other lines in the poem use the same technique, so that they become like members of a funeral procession marching in slow, steady, succession.
The poem also uses strategically placed punctuation to force us to pause and linger over certain moments, like that comma after "mine," in line 5. We just have to stop and take a moment of silence in honor of Milton's deceased wife. The poem is full of commas (and one exclamation point) that break up the flow of the lines. It's as though the poem is telling us to slow down and really linger over the vision of the "late espoused Saint," as the speaker no doubt wants to.
Taken together, the poem's language and concepts, ponderous, slow-moving meter, and punctuation make it sound important and grand—even stately.