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Why do Roman Catholics consider St. Peter the first pope?

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Mosaic: Christianity — Christianity Christ as Ruler, with the Apostles and Evangelists (represented by the beasts). The female figures are believed to be either Santa Pudenziana and Santa Práxedes or symbols of the Jewish and Gentile churches. Mosaic in the apse of Santa Pudenziana basilica, Rome, 401–417 ce.

Roman Catholics consider St. Peter the first pope based on the Petrine theory, a theological construct rooted in the belief that Jesus Christ explicitly appointed Peter as his vicar on Earth and the visible head of the church.12

This tradition relies primarily on passages from the New Testament, most notably Matthew 16:18–19, where Jesus tells Peter, "You are Peter, and upon this rock I will build my church," and grants him "the keys to the kingdom of heaven". Additionally, John 21:15–16 is cited, in which Jesus instructs Peter to "feed my lambs" and "tend my sheep".32

The Roman Catholic Church interprets these texts to mean that Christ established a primacy of jurisdiction that was intended to pass in perpetuity to Peter’s successors as the bishops of Rome. While this is a foundational doctrine of the faith, Catholic scholarship acknowledges that the identification of Peter as the first "pope" is a later theological development, as the formal office of the papacy did not exist in the 1st century in the way it evolved historically.421