The scepter of Your kingdom is a scepter of uprightness.
This is a straight scepter, because Christ's teaching and the Laws of His kingdom are altogether straight and pure, namely, that we should believe in Christ and love God and our neighbor. There is no error in this Law, but there is in us; for we do not believe enough, we do not love enough, we are not strong enough in tribulation. And yet, since we are under this shadow of Christ, who loves God and man purely, we enjoy His favor and are regarded as holy also in this life. First, then, there is no error in His Law, no crookedness in His scepter. For the Word is pure, promising grace and teaching men to love God. This is the equity of His scepter. Second, there is no error in us either: first, so far as concerns our Head, Christ, who absorbs our sins; and second, so far as concerns the future life, in which the righteousness will be revealed that we now believe.
From Lectures on Psalm 45 (Luther's Works 12:239-40)