To be specific, it was Constantine. In his use of Christianity to unify his armies, he adopted the Mithra Sun worshipers holy day to make it incompatible and kill off what was then the strongest local religion competition.
Unity of church and state brought this change to the Roman church, and its philosophical descendants.
Gentile christians worshipped on Sunday back in the 2nd century, long before Constantine.
Sure, he might have made it official, but he did not start it.
Yes, I was agreeing and adding an important detail.
It wasn't just gentile Christians using Sunday.
Jewish Christians also kept sabbath and then met with Christians on Sunday also, according to Ignatius and Barnabas at al.
https://archive.gci.org/articles/sabbath-and-sunday-in-the-early-church/
