He, viz. Constantine The Great, was quite the guy.
Was a pagan all of his life, only agreeing to be baptised on his deathbed when he was 65 (in 337 AD).
‘Battle of the Milvian Bridge’, where he saw THE SIGN in the sky took place in 312.
The ‘Edict of Milan‘, which made Christianity an officially recognised and tolerated religion in the Roman Empire came in 313.
This edict making Sunday, into Sunday in terms of how we know it, came 8-years later.