We know from history that tens of thousands of orthodox Jews, (right in the heart of Jerusalem where their claims could be easily refuted if not true), made the Resurrection their central message, abandoned Saturday as their holy day, and accepted Sunday (the day of the Resurrection) as their new holy day.
Know is a strong word.
Why would people just up and submit to the harshest abuse possible unless for something they knew to be false?
Why would people (i.e. "tens of thousands of orthodox Jews") convert to Christianity, submitting themselves to harsh abuse, for something they knew to be false (i.e. the story of Jesus death and resurrection)?
Before we answer the question we must be aware that the "tens of thousands" of Jewish converts in Jerusalem in the first century CE is a number purported once again by the Bible. Not all scholars accept this number. Some believe the number of converts in the Bible is highly exaggerated.
Some scholars (see: Hopkins, K 1998. Christian number and its implications. Journal of Early Christian Studies 6, 185-226) postulate that the Jewish converts in Jerusalem never abandoned their faith in Judaism, Jewish tradition, Jewish worship, or believed the Torah no longer had to be followed. In addition, scholars also have concluded that the number of Jews in Jerusalem in the first century CE never exceeded approximately 1,000 people.
Rather, there seemed to be 2 churches in the first century CE: the Jews in Jerusalem and the Hellenistic non-Jews whom Paul converted. The Hellenistic Christians quickly abandoned Jewish scripture and practices. This caused conflict between the 2 groups.
What information do we have about the first century CE Roman Empire? The Roman cultural was heavily influenced by its predecessors, the Greeks. Greek religion influenced Roman religion. Both had polytheistic religions with humanized gods (The gods had physical bodies and would often visit humans on Earth). Polytheistic religions tend to be accepting of other religions and gods.
What seems more plausible?
1. There were tens of thousands of Jews in Jerusalem who accepted Jesus as the messiah (the Biblical Jesus did not fulfil all the prophecies of the messiah in the Torah), accepted that Jesus died, was buried, and resurrected himself, rejected the Torah and Jewish religion to worship on Sunday or
2. There were thousands of non-Jewish converts to Paul's ministry who were never Jews, were raised in a culture/religion that freely accepted other religions (even an altar to an unknown god), other gods, gods who frequently took human form to interact with other humans, and were even killed only to be resurrected (see: Asclepius Greek god of Medicine who Zeus feared would make man immortal only to be resurrected to Olympus).