On the Morning of Christ's Nativity
deadly forfeit
Legal language—humanity owes a death penalty for sin. Christ "releases" us from the debt, like paying off a bond or ransom.
Trinal Unity
The Trinity. Milton pictures Christ at God's council table before incarnation—he's describing what Christ gave up by becoming human.
star-led Wizards
The Magi. "Wizards" meant wise men or astrologers in 1629—no Harry Potter connotation. They navigate by the star.
prevent them
"Prevent" = arrive before, not stop. Milton wants his poem to reach Christ before the Magi's gifts do—a race to worship first.