To be taxed. — Literally, to register himself.
With Mary his espoused wife. — Many of the best MSS. omit the substantive: “with Mary who was betrothed to him.” The choice of the participle seems intended to imply the fact on which St. Matthew lays stress (Matthew 1:25). She went up with him, not necessarily because she too had to be registered at Bethlehem, but because her state, as “being great with child,” made her, in a special sense, dependent on Joseph’s presence and protection.