Hello, I am making an alternate history video and was wondering what would happen if someone was not Christian during Constantine's reign. Was this the only religion allowed? Was the traditional Roman god pantheon also allowed? What would punishments look like for a Pagan or someone of any other religion?
Short answer: yes. Long answer, although Constantine is considered a Christianising emperor, he wasn't overtly Christian to the point of direct persecution against pagans. Constantine was actually involved in what we may consider 'pagan practises' (imperial cult, claiming descent from pagan divinities etc). Christianity became much more... forced with Theodosius the Great, with active persecution of pagans etc.
I would recommend checking out Rodgers, B.S. 1980. “Constantine’s Pagan Vision.” Byzantion. 50:1. 259-278. She has really interesting work and there's a few across the years - definitely check them out if you're interested in a broader scope.
Additionally, this article is really good at looking at how the two religions intersected, particularly in 'the early days': Marsengill, K. 2015. “The Visualization of the Imperial Cult in Late Antique Constantinople”. Jefferson, L.M., Jensen, R.M. (eds.) Art of Empire: Christian Art in its Imperial Context. Fortress Press, Mineappolis. (271-305).
I'm sorry this doesn't completely answer your questions - I had the same and it's a much bigger question than it seems (which is frustrating). These sources should hopefully provide some more substance.