My money would be on "Viva Cristo Rey", but that's Spanish instead of Latin. Plus it's not actually in Scripture.

However, the closest thing in Scripture would be

1 Corinthians 15:25
Oportet autem illum regnare donec ponat omnes inimicos sub pedibus ejus.
Which translates:
For he must reign, until he hath put all his enemies under his feet.

For the sake of brevity, he could probably abbreviate it, "Oportet illum regnare" which means he should/must reign, or it behooves him to reign, or it is fitting that he should reign.

Autem is just a conjunction, rendered various ways (but, as for, and)

Basically it's talking about how Christ should be King. You know, "Christ the King".

Viva Cristo Rey!