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?"
Apostle Paul
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
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 Roman Empire.
Paul the Apostle, Emporer Constantine, and others
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 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
The spread of Christianity in the Roman Empire evolved over time from a small religious movement to becoming the official religion of the empire. This evolution was influenced by factors such as the conversion of Emperor Constantine, the Council of Nicaea, and the missionary efforts of early Christian leaders. Ultimately, Christianity's growth and acceptance reshaped the religious landscape of the Roman Empire and beyond, leading to the establishment of Christianity as a dominant and enduring faith in the fullness of time.