Bart Ehrman, The New Testament: A Historical Introduction to the Early Christian Writings, sixth edition (OUP, 2016), 83-84.
Without a doubt, the most important thing that was happening for early Christianity was the spread of the religion from its inauspicious beginnings as a tiny sect of Jesus’ Jewish followers in Jerusalem – the Gospels indicate that there were eleven men and several women who remained faithful to him after his crucifixion, say, a total of fifteen or twenty people altogether – to its status as a world religion enthusiastically supported by Christian believers throughout the Roman Empire. Missionaries like Paul actively propagated the faith, converting Jews and Gentiles to a belief in Christ as the Son of God, who was crucified for the sins of the world and then raised by God from the dead.
By the end of the first century, the tiny group of Jesus’ disciples had so multiplied that there were believing communities in cities of Judea and Samaria and Galilee, probably in the region east of the Jordan River; in Syria, Cilicia, and Asia Minor; in Macedonia and Achaia (modern-day Greece); in Italy; and possibly in Spain. By this time Christian churches may have sprung up in the southern Mediterranean, probably in Egypt and possibly in North Africa.