Protestantism did not exist until Martin Luther and John Calvin and the Reformation of the 16th century, long after the age of paganism.
You obviously have your work cut out for yourself, trying to get up to speed on the history of Christianity.
In the first three centuries the first Christians were called, converts to Judaism before they were called Christians.
Judaize
YLT
Galatians 2:14
But when I saw that they are not walking uprightly to the truth of the good news, I said to Peter before all, `If thou, being a Jew, in the manner of the nations dost live, and not in the manner of the Jews,
how the nations dost thou compel to Judaize?
Galatians 2:14
But when I saw that they were not walking uprightly according to the truth of the good news, I said to Peter before everyone, If you, being a Jew live in the manner of the nations, and not in the manner of the Jews, then
how will you compel the nations to Judaize?
Sabbath Judaizers
The Council of Laodicea of around 365 AD decreed 59 laws, #29:
"Christians must not judaize by resting on the Sabbath, but must work on that day, rather honoring the Lord's Day; and, if they can, resting then as Christians. But if any shall be found to be judaizers, let them be anathema from Christ."