Commentary Par XII 40

That God is here referred to as 'emperor' (as He is on only two other occasions: [Inf I 124] and [Par XXV 41]) makes Dante's comfort with imperial trappings clear, especially to his Guelph enemies.  This term for God is not in itself unwarranted in Christian tradition, far from it.  But Dante uses it here in an ecclesiastical context where it might seem, at least to some, improper.