The foundation of Christianity was indeed laid during Jesus’ earthly ministry, through His teachings, miracles, death, and resurrection. The reason Christianity began to spread more widely after His death and resurrection is deeply rooted in the fulfillment of His mission and the establishment of the New Covenant.

Jesus came to fulfill the prophecies of the Old Testament, proclaiming the arrival of the Kingdom of God, offering salvation to all through His death and resurrection, and establishing a new covenant between God and humanity. “For this is my blood of the covenant, which is poured out for many for the forgiveness of sins.” (Matthew 26:28) His resurrection was the pivotal event that confirmed His divine nature and the truth of His teachings to His followers.

Before His ascension, Jesus instructed His disciples to spread His teachings to all nations, baptizing them in the name of the Father, and of the Son, and of the Holy Spirit, teaching them to observe all that He had commanded them. (Matthew 28:19-20) However, it was the coming of the Holy Spirit at Pentecost that empowered the early followers to begin preaching the gospel with boldness. “But you will receive power when the Holy Spirit has come upon you, and you will be my witnesses in Jerusalem and in all Judea and Samaria, and to the end of the earth.” (Acts 1:8)

The spread of Christianity after Jesus’ ascension was therefore a fulfillment of His mission and command. The early church, led by the apostles and empowered by the Holy Spirit, began to preach the gospel, leading to the formation of Christian communities among Jews and Gentiles alike. The letters of the New Testament reflect the ongoing development of Christian theology and church practice as the apostles worked to spread Jesus’ teachings and nurture the growing Christian movement.

In essence, Christianity was formed during Jesus’ earthly ministry but manifested and spread widely after His resurrection in accordance with His divine plan and the empowerment of His followers by the Holy Spirit.