Greco-Roman art focused on realism as the individual human became more valued in their culture. Christianity valued humans as a flock that faithfully followed Christ as their shepherd. I do believe in God and Jesus, but I also believe that some teachings of the bible were meant to keep the lower class in line.
Early Christians were trying to invent their own art style that would be separate in ways from the Greco-Roman foundation that they had emerged from. The resulting images of humans and Christ ended up abstracted, more like images in an instruction manual than figures in a photo. I think something that contributed to the abstract nature of Byzantine art was that the people illuminating manuscripts were monks locked away underground, not really out looking at the landscapes they were trying to recreate. Some of the abstraction was for the purpose of clear communication though, like in comic books.
Iconoclasm wasn’t good for the continued development of art — it seems to have set artistic ability back a few steps. Artists had to work in secret on books that contained images, and probably were illustrated by novices who had no great masters left to teach them. This is like the interruption of Egyptian art by being conquered.