It was Peter as he was crucified in Rome.
The apostle Paul spread Christianity throughout the Mediterranean regionSaint Patrick spread Christianity throughout Ireland!
Christianity did not easily spread through the Roman Empire. See the related question, "How did Christianity spread throughout the Roman Empire?"
The spread of Christianity had political ramifications for the Roman Empire. The empire split into two, Constantinople to the east and Rome to the west
Apostle Paul
By 325ad Christianity had spread throughout the Roman Empire and to Britain. If you google "spread of Christianity map" you will find maps showing the spread.
Christianity
The Europeans and their descendants elsewhere are Christian because of the Roman Empire. Judea was a province of the Roman Empire in Jesus Christ and the Romans crucified Jesus. Christianity spread around the Roman Empire, became the religion of the masses in this empire and then mainstream Christianity became the state religion of the empire. What was to become Catholicism spread and developed in the western part of the empire and was originally called Western or Latin Christianity. The Popes became its heads during the Roman days. Orthodox Christianity spread and developed in the eastern part of the Roman Empire. Originally it was called Eastern of Greek Christianity. The Patriarchs of the Orthodox Church already existed in the Roman days.
The Roman Empire.
Paul the Apostle, Emporer Constantine, and others
In Christianity, an apostle is typically understood to be one of the twelve disciples chosen by Jesus Christ to spread his teachings. Apostles are considered the foundation of the early Christian church and are thought to have been personally commissioned by Jesus. Their role was to establish and spread the teachings of Jesus, often through missionary work and writing letters to different communities.
The Roman Empire made Christianity the official religion of the Empire, and 'encouraged' the peoples it conquered to convert.
The Romans who remained pagans did not contribute to the spread of Christianity Neither did Julian, who was the only pagan emperor in the Later Roman Empire