The tribes of Juda and Levi were united by a fusion of their lines of descent and that is why Matthew assigns Christ's family to the tribe of Juda. And the Apostle says, 'for our Lord has sprung out of Juda' (Heb. 7:14). Thus, from the tribe of Levi may be counted a heritage that is priestly and filled with holiness, while from the tribe of Juda - to which David and Solomon and the rest of the kings belonged - there shines forth the splendor of a royal decent. And so, by the testimony of the Scriptures, Christ is shown to be at once both king and priest.
St Ambrose of Milan. Seven Exegetical Works.