Sol Invictus — Grokipedia

Etymology and Pre-Imperial Associations
The Epithet Invictus in Roman Religion
Early Solar Deities and Syncretism
Establishment and Imperial Promotion
Elagabalus's Solar Cult Attempts (218–222 AD)
Aurelian's Reforms and State Cult (274 AD)
Continuation under Constantine and Successors
Constantine's Syncretism and Symbolism (306–337 AD)
Sol Invictus in the Late Empire (Post-Constantine)
Cult Practices and Iconography
Temples, Priesthoods, and Rituals
Festivals and the Dies Natalis Solis Invicti (December 25)
Theological Dimensions
Syncretism with Mithras, Apollo, and Eastern Gods
Debates on Henotheism versus Polytheism
Decline and Suppression
Rivalry with Emerging Christianity
Theodosian Decrees and Pagan Persecutions (late 4th century)