The bishop Theodulf of Orléans (~760 - 821 CE) was a theological and legal advisor to Charlemagne, perhaps number two after Alcuin. Theodulf wrote a number of influential works, often in poetic format, about Carolingian Frankish law and justice. Chief among these is the poem titled Comparison of Ancient and Modern Law. Not only were early medieval kings referring to ancients, but also to themselves as 'modern'. How distant the 'ancients' were in the mentalities of early medievals is something medievalists argue about, but certainly back through the Old Testament, and in a peculiar thread, to the Trojans. The mythic origins of Gallo-Romans descended from Trojans became a solid, reliable trope through the Merovingians that continues to appear in Carolingian literary works. Also of some debate is exactly what early medieval use of 'modern' connoted; it seems to not be a sense of progress of civilisationas we would have it, but 'current' or 'contemporary'.

For a thorough treatment of how western medievals inherited, related to and used the past:

• Chris Wickham, Inheritance of Rome: Illuminating the Dark Ages 400-1000 (Penguin, 2009)

Also some interesting but more academic essays here:

• Yitzhak Hen and Matthew Innes, Uses of the Past in the Early Middle Ages (Cambridge University Press, 2000)

I'm going to leave it to specialists in Late Antiquity to discuss the use of 'ancient' by rulers and ecclesiastics before the medieval period.