This phrase, parallel to the one preceding, marks the poetic style of this song. Compare also the parallel phrases in v. 2.
Must be just.
The “must be” has been supplied. The sense of the Hebrew is, “he that ruleth righteously.” The blessedness of a righteous ruler is extolled rather than a ruler being admonished to rule righteously.
In the fear of God.
“The powers that be are ordained of God,” and he that holds office is a “minister of God” (Rom. 13:1, 4). Every man who rules should, therefore, do so in the constant fear of God, always conscious that he rules by divine appointment and that Heaven holds him responsible for his every decision.