The speaker of "Methought I Saw my Late Espoused Saint" uses three major similes to describe his wife as she appears in his vision: the Greek myth of Alcestis, a woman rescued from the underworld by Hercules; an Israelite woman ritually purified from childbirth according to the laws in Leviticus; and, finally, the woman he believes he'll see again in Heaven. The progression of these similes moves from ancient pagan beliefs, to those of the Hebrew Bible, to Christian beliefs surrounding death and the afterlife, which is Milton's handy way of retelling the history of Christian salvation in the briefest way possible. But the similes also draw comparisons between pagan, Judaic, and Christian belief systems, too. According to the speaker of the poem, the Christian faith is the way to go.
The progression of similes is Milton's way of saying that Christian salvation is preferable to all the religious beliefs that came before it.
The speaker's beliefs don't mean much here, because he's not actually seeing his wife, the saint. He has no idea where she has gone after death, so his religious convictions don't hold much water.