Early Christianity is the period of Christianity preceding the First Council of Nicaea in 325. It is typically divided into the Apostolic Age and the Ante-Nicene Period (from the Apostolic Age until Nicea). 

The first Christians, as described in the first chapters of the Acts of the Apostles, were all Jews either by birth or conversion, for which the biblical term "proselyte" is used, and referred to by historians as Jewish Christians. The early Gospel message was spread orally, probably in Aramaic, but almost immediately also in Greek. The New Testament's Acts of the Apostles and Epistle to the Galatians record that the first Christian community was centered in Jerusalem and its leaders included Saint Peter, James, the brother of Jesus, and John the Apostle. 

After the conversion of Paul the Apostle, he claimed the title of "Apostle to the Gentiles". Paul's influence on Christian thinking is said to be more significant than that of any other New Testament author. By the end of the 1st century, Christianity began to be recognized internally and externally as a separate religion from Judaism which itself was refined and developed further in the centuries after the destruction of the Second Temple. 

Numerous quotations in the New Testament and other Christian writings of the first centuries, indicate that early Christians generally used and revered the Hebrew Bible (the Tanakh) as religious text, mostly in the Greek (Septuagint) or Aramaic (Targum) translations.
Were the first Christians Jewish?
were all Jews either by birth or conversion