Complex sentence: Sun and Moon cults were probably represented at the Circus from its earliest phases but their importance grew with the introduction of Roman cult to Apollo, and the development of Stoic and solar monism as a theological basis for the Roman Imperial cult.
Split into simpler sentences: Sun and Moon cults were probably represented at the Circus from its earliest phases. Their importance grew with the introduction of Roman cult to Apollo, and the development of Stoic and solar monism as a theological basis for the Roman Imperial cult.