You are by far over stating the number of Jews that became Christians in the early history. Within a generation or two Paul was was widely teaching throughout Rome and the majority of Christians were not of Jewish background. By about 200 to 400 AD when the New testament evolved toward its present form the remaining Jews were a very small minority, and the Church Fathers were almost entirely Roman and Greek.
I dont think a generation or 2 concerning Paul.
He died in 65 AD.
Nero killed thousands of Christians in his blame for setting fire to rome.
Josephus, Pliny the younger, Tacitus (Nero historian who investigated the Roman fire), Polycarp, Ignatius, all describe the Tribe of Christians.
if you were to claim that the Christians was some small sect in a corner of the world to ignore, you are wrong. There were studies made on graves with Christian references, and by the end of the First century the Christian religion was flaring out all over the known world.
Think about this.
When Jesus walked in palestine, whole villages and towns came to listen him.
about 10 000 to 15 000 were present at his teachings, not only once, but numerous times.
he was a huge problem to the sanhederen and he hurt their moneymaking scams terribly.
He actually almost destroyed their religious money making practices, and they had to take him out.
Little did they know they were actually assisting in spreading the religion we today know as Christian.
Please take note, Paul and the Apostles went all over Greece and Rome, preaching in Synagogues.
The Jews became Christians first, then the religion was spread to those who were not Jewish.
The Jews actually shaped the Christian religion by their scriptural knowledge they had, and spoke by authority from the Bible to the newly converts.
There are numerous evidence to attest that there were a big number of Christians before the first century passed off.
The oldest Christian church?
Already at jamnia the Jews decided the Christians are not Jews. 85 AD.
Nerva also declared that the Christians were not jews and did not have to pay Jewish taxes.
I do not see how anyone can think that the Christians were a very small number of people in the days from jesus to 100 AD.
No one would have bothered if they were even 10 000 in 65 AD.
The Emperors and Jews would certainly have noticed 50 000 and more as a threat to Rome and Jerusalem.