"three lives in one" (line 10) from the poem "The Flea" by John Donne. How does this line give reference to the Christian concept?

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.)