The Christian theology in the Divine Comedy is the whole concept of heaven, hell, and purgatory. He also places people like Plato and Socrates in Limbo because they did not believe in God/ were never baptized. He alludes to Classical mythology by including the River Styx and Acheron. He also places people like Helen, Achilles, Dido and Paris in the Inferno. He also includes Minos in the second circles of Hell. I think Dante included both because he had a great respected for the two cultures. I also thought maybe he himself was referencing his journey with his faith.
I agree! Dante seems to know very well and seems really attached to the Ancient Greece literature. I also believe that Dante’s poetry skills are built from the Greek literature so he found it inevitable to skip those characters and not use them in his work. His belief/imagination together with his literature skills perfectly included and played with every character throughout his work.