Finding this imagery in the Book of Genesis, the Allegory images authentic spiritual authority as the Sun and any and all civil, or political or secular, authority as the Moon. By doing so, it illustrates that the Roman Catholic Pope, as "Supreme Pontiff", "Vicar of Christ", et cetera, and therefore the supreme universal spiritual authority on Earth, is like the Sun that is the one source of light for itself and all other celestial bodies orbiting it; while the Holy Roman Emperor, as symbolic and intended supreme civil, political, and secular authority on Earth, and having theoretically received his authority from and at the pleasure of the Pope, is like the Moon - that is, dependent upon the Sun for any illumination, merely reflects solar light, and ultimately has no light without the Sun. This theory dominated European political theory and practice in the 13th century. It is related to the general theory of Papal supremacy and "plenitudo potestatis" as articulated by the Roman Catholic Church.
Pope Innocent III's Letter to the prefect Acerbius and the nobles of Tuscany, 1198,
Just as the Founder of the universe established two great lights in the firmament of heaven, the greater light to rule the day and the lesser light to rule the night, so too He set two great dignities in the firmament of the universal Church..., the greater on to rule the day, that is, souls, and the lesser to rule the night, that is, bodies. These dignities are the Papal authority and the royal power. Now just as the Moon derives its light from the Sun and is indeed lower than it in quantity and quality, in position and in power, so too the royal power derives the splendor of its dignity from the pontifical authority....
In 1215, this concept was reflected in Canon 3 of the Fourth Lateran Council regarding heretics:
...if a temporal ruler, after having been requested and admonished by the Church, should neglect to cleanse his territory of this heretical foulness, let him be excommunicated by the metropolitan and the other bishops of the province. If he refuses to make satisfaction within a year, let the matter be made known to the supreme pontiff, that he may declare the ruler's vassals absolved from their allegiance and may offer the territory to be ruled by Catholics, who on the extermination of the heretics may possess it without hindrance and preserve it in the purity of faith; the right, however, of the chief ruler is to be respected as long as he offers no obstacle in this matter and permits freedom of action.
While this theory of papal sovereignty in temporal as well as spiritual matters was, by the fourteenth century, generally rejected as out-of-date, it entered into canon law and was reinforced by the Allegory of the Two Swords in the bull of Pope Boniface VIII entitled Unam Sanctam. Dante Alighieri further expounded and argued in favor of the Allegory of the Sun and Moon in his De Monarchia and other works, but he emphasized the dignity of the offices of both the Pope and the Holy Roman Emperor and the harmony they should have in the execution of their obligations due to the shared ultimate objective of saving souls via the one true Roman Catholic Church.
