Final answer:
The birth of the Christian church is often associated with the Pentecost, although other critical events include Apostle Paul's missionary work and Roman Emperor Constantine's conversion to Christianity and subsequent legalization of the religion, which fostered its spread and establishment as a major religious institution.
Step-by-step explanation:
The event that could fairly be described as the birth of the Christian church is the Pentecost, which took place after Jesus' resurrection and ascension. This event marked the descent of the Holy Spirit upon the apostles and is seen as the moment when the apostles were empowered to begin their ministry of spreading Jesus' teachings.
However, in a broader historical context, the conversion of the Apostle Paul and his subsequent missionary work to spread Christianity to the Greco-Roman world can also be identified as critical in the expansion of early Christianity, establishing communities and an identity separate from Judaism.
Another momentous event was when the Roman Emperor Constantine underwent a conversion experience to Christianity in the early 4th century and decreed the legalization of Christianity, followed by the Emperor Theodosius' declaration of Christianity as the official state religion later that century.
These events played pivotal roles in integrating Christianity into the social and political fabric of the Roman Empire. The development of church hierarchy, rituals such as baptism, the establishment of core theological principles by thinkers and councils, and the creation of a unified Christian narrative all contributed to the structured growth of the Christian church.