Now as to the kind of performances peculiar to the circus exhibitions. In former days equestrianism was practised in a simple way on horseback, and certainly its ordinary use had nothing sinful in it; but when it was dragged into the games, it passed from the service of God into the employment of demons. Accordingly this kind of circus performances is regarded as sacred to Castor and Pollux, to whom, Stesichorus tells us, horses were given by Mercury. And Neptune, too, is an equestrian deity, by the Greeks called Hippius.
In regard to the team, they have consecrated the chariot and four to the sun; the chariot and pair to the moon. But, as the poet has it, “Erichthonius first dared to yoke four horses to the chariot, and to ride upon its wheels with victorious swiftness.” Erichthonius, the son of Vulcan and Minerva, fruit of unworthy passion upon earth, is a demon-monster, nay, the devil himself, and no mere snake. But if Trochilus the Argive is maker of the first chariot, he dedicated that work of his to Juno.
If Romulus first exhibited the four-horse chariot at Rome, he too, I think, has a place given him among idols, at least if he and Quirinus are the same. But as chariots had such inventors, the charioteers were naturally dressed, too, in the colours of idolatry; for at first these were only two, namely white and red—the former sacred to the winter with its glistening snows, the latter sacred to the summer with its ruddy sun: but afterwards, in the progress of luxury as well as of superstition, red was dedicated by some to Mars, and white by others to the Zephyrs, while green was given to Mother Earth, or spring, and azure to the sky and sea, or autumn.
But as idolatry of every kind is condemned by God, that form of it surely shares the condemnation which is offered to the elements of nature.
Source: De Spectaculis (The Shows) (New Advent)