https://watv.org/bible_word/biblical-sabbath-is-saturday/
To the Romans, Christianity seemed to be a sect of Judaism because it was also a monotheistic religion; Christians believed in Jesus as the Savior, and kept the seventh day as the Sabbath just as the Jews did. So, the people of the Church in Rome were unable to avoid persecution from the Romans, and they made efforts to inform them of the difference between Christianity and Judaism. In this process, they came to accept Sunday, which the Romans observed, as the day of worship.
Sunday was the day set aside for the worship of Mithra, the sun god. Mithraism, an offshoot of the Persian religion Zoroastrianism, was introduced to Rome around the 1st century B.C. Mithra was described as the”invincible god” or the”god of immortal youth,” and Mithraism had a following mostly among the soldiers of Rome.
Around the time when Christianity spread in Rome, Mithra the sun god had already been raised to the status of the patron deity of the emperors and the empire, and Mithraism had been recognized as the most predominant and influential religion in Rome. The Church of Rome tried to escape persecution from the Romans by accepting Sunday, which was kept in honor of Mithra, as the day of worship.