Vatican Necropolis in the context of "Christianity in the ante-Nicene period"

Christianity in the ante-Nicene period was the period in Christian history following the Apostolic Age (1st century AD) up to the First Council of Nicaea (325 AD). Although the use of the term Christian (Koine Greek: Χριστιανός) is attested in the Acts of the Apostles (80–90 AD), the earliest recorded use of the term Christianity (Koine Greek: Χριστιανισμός) is attested by the ante-Nicene Father and theologian Ignatius of Antioch (c. 107 AD).

While the Jewish–Christian community was centered in Jerusalem in the 1st century AD, Gentile Christianity spread widely in the 2nd century AD. One stream of Gentile Christianity (so-called "proto-Orthodox Christianity") that emerged in this period in the persons and theological positions of the Apostolic Fathers would eventually become the international Great Church. Proto-Orthodox Christianity placed importance on the sacrifice of Jesus on the cross as saving humanity, and described Jesus as the incarnated Son of God come to Earth. The 2nd and 3rd centuries AD saw a sharp separation between Jewish Christianity and Gentile Christianity, with the latter being derived from the teachings of the Apostle Paul. There was an explicit rejection of Second Temple Judaism and Jewish culture by the end of the 2nd century, with a growing body of anti-Jewish Christian literature. Many doctrinal variations in this era defy neat categorizations, as various forms of Christianity interacted in a complex fashion.

The popularly named "Tomb of the Julii" (Mausoleum "M") survives in the Vatican Necropolis beneath St. Peter's Basilica. The serendipitous discovery near the crypt has a vaulted ceiling bearing a mosaic depicting a solar deity with an aureole riding in his chariot, within a framing of rinceaux of vine leaves.

The surrounding Christian iconography, such as other mosaics in this tomb depicting Jonah and the whale, the good shepherd carrying a lamb (the kriophoros motif), and fishermen have led to an interpretation of the deity as Christ, known in this form as "Christus-Sol" or "Christ-Apollo". Due to these symbols, the tomb is interpreted as an early Christian vault. Allen Brent has described it as a "synthesis between Christ and Apollo", in the context of Emperor Aurelian's promotion of Sol Invictus as the chief god of the Roman Empire. According to Brent, Sol Invictus - "with whom Apollo also could be identified" - represented the "shared deity of all individual gods", and the Christian creators of the Tomb of the Julii wished to "express the metaphysical order in the cosmos in terms of a Christian syncretism".

The Vatican Grottoes are a series of underground chambers and chapels located under part of the nave of St. Peter's Basilica in the Vatican. They are situated three meters below the current floor, and extend from the high altar (the papal altar) to about halfway down the aisle, forming a true underground church that occupies the space between the current floor of the Basilica and that of the old Constantinian basilica of the 4th century.

The Vatican Grottoes are not to be confused with the Vatican Necropolis, which is an ancient burial ground that dates back to the Roman times and lies at a lower level beneath the Grottoes and the Basilica.

Saint Peter's tomb is a site under St. Peter's Basilica that includes several graves and a structure said by Vatican authorities to have been built to memorialize the location of Saint Peter's grave. The site of St. Peter's tomb is alleged to be near the west end of the Vatican Necropolis, a complex of mausoleums that date between about AD 130 and AD 300.

The Necropolis complex was partially torn down and filled with earth to provide a foundation for the building of the first St. Peter's Basilica during the reign of Constantine I in about AD 330. As the result of two campaigns of archaeological excavation, many bones have been found at the site of the 2nd-century shrine, but Pope Pius XII stated in December 1950 that none could be confirmed to be Saint Peter's with absolute certainty.