"marriage temple" (line 14) from the poem "The Flea" by John Donne. How does it become elements of spiritual in the poem?

In the second stanza the poet argues for the life of the flea, as his desired lady has made a move to kill it. He paints the flea as a holy thing: “This flea is you and I, and this/Our marriage bed, and marriage temple is” (lines 12-13). (Note also the reference to the Christian concept of "three lives in one" (line 10), suggesting that a spiritual union already exists, although unlike a spiritual marriage in a "marriage temple," the third being in the trio is not God but a flea.) Besides arguing for the sanctity of the flea’s life, the speaker is also arguing that he and the lady have already bypassed the usual vows of fidelity and ceremony of marriage; thus, he pushes toward his point that the two of them have already been joined as one in the flea, so there is no harm in joining their bodies in sexual love.