Final answer:
The early church's growth was due to factors such as evangelism by figures like Apostle Paul, larger building constructions for worship, and Constantine's conversion which tied the religion to the Roman power structure, attracting many elites.
Step-by-step explanation:
The significant expansion of the early church in the first three decades can be attributed to several factors. Evangelism, the act of spreading the Christian message, was a powerful tool used by early Christians, most notably by the Apostle Paul as he carried Christianity beyond Jewish communities into the Greco-Roman world. After the end of persecution, Christians had the opportunity to construct larger worship buildings, which were often funded by the large crowds that gathered on holy days. These structures served not only as places of worship but also as symbols of the growing influence of Christianity.
The conversion of Constantine, which is considered one of the critical moments in Christian history, had a profound impact. His conversion led to imperial sponsorship, which shifted the perception of Christianity from a commoners' belief system to one that was integrated with the Roman power structure. Consequently, converting to Christianity became a means for advancement within the empire, which naturally attracted many Roman elites. This 'top-down' influence considerably bolstered the early church's growth and influence.
Moreover, the early Christian church's organizational abilities, dedication to missionary work, and the development of church buildings capable of accommodating expanding congregations also contributed to its growth. Churches began to feature grand architecture and decorative elements, which reflected and reinforced the power and stability of the Christian faith during this period.