Besides the paganism of Mithraism, Sunday worship (1st day of the week) also came to be from misguided Christians who at the time, viewed Sunday as a little "Easter" each week.
One can see how these 2 forces-external paganistic Mithraism & internal "Easter" Sunday emphasis bonded in the Roman era of Church history resulting in Sunday sacredness in place of Sabbath sacredness.
Of course---the Scriptures never changed as far as God is concerned.... But the seventh day is the Sabbath of the Lord thy God: Exodus 20:10
One can see how these 2 forces-external paganistic Mithraism & internal "Easter" Sunday emphasis bonded in the Roman era of Church history resulting in Sunday sacredness in place of Sabbath sacredness.
Of course---the Scriptures never changed as far as God is concerned.... But the seventh day is the Sabbath of the Lord thy God: Exodus 20:10