Saint : My late espousèd saint : Katherine Woodcock, Milton’s second wife, who had died shortly after giving birth to a baby boy. Milton had never in fact seen her, as he had gone blind before their first meeting.

Alcestis : Heracles went into the underworld and wrestled with death to bring Alcestis, the wife of King Admetus, back from the dead.

Jove’s great son : Heracles.

Purification in the old Law :
Leviticus 12, 2-4
2. If a woman be delivered, and bear a man-child, then she shall be unclean seven days; as in the days of the impurity of her sickness shall she be unclean. 
3 And in the eighth day the flesh of his foreskin shall be circumcised. 
4 And she shall continue in the blood of purification three and thirty days; she shall touch no hallowed thing, nor come into the sanctuary, until the days of her purification be fulfilled. 
The strictures with regard to a female child were more elaborate. Milton was at least spared the necessity of sacrificing a goat. He is here presumably congratulating himself on having done everything necessary before she died to secure her entrance into heaven, where he is confident about meeting her again in due course.