Methought I saw my late espoused saint (1)
Calling his wife a saint tells us that his wife was really virtuous, and a faithful Christian, on earth. She'd have to be in order to get into heaven, which is where calling her a "saint" implies that she is.
Come to me like Alcestis, from the grave (2)
Alcestis was a Greek mythological figure who sacrificed her life for her husband's and was rescued from the underworld by Hercules. Plato's Symposium, with which Milton was no doubt familiar, upholds the figure of Alcestis as an example of the highest form of love. What's the point of all this? Well, comparing her to Alcestis suggests that his wife possessed this kind of love for him—perfect love. It also makes us wonder if the speaker views his wife as having died for him in some way.
Mine, as whom wash't from spot of child-bed taint Purification in the Old Law did save. (5-6)
Leviticus 12 required a woman to undergo a waiting period and a ceremony of cleansing before re-entering the community after the birth of a child, from whose taint she was then said to be "cleansed." We know that Milton's second wife, Katherine Woodcock, probably died from complications related to childbirth, so that this comparison links the woman in the vision to her.