Rodney Stark isn't a Catholic, but his history "The Triumph of Christianity" is phenomenal. Even better sourced: Paul Johnson's "The History of Christianity". (Johnson is a Catholic, but he leaves no rock unturned and has no inhibitions about criticizing the Church). I'd say if neither of those two books convinces you, you'll remain convinced of your current position.

To the extent that this massive question and even greater answer can be summarized, I'd say that the Peterine and Pauline "Jesus Movement" (i.e. the Judaic and largely Hellenic groups) converged in large part as a result not just of the persecutions under Nero and Diocletian, but also because of the brutal Persian persecution of the church. I'd ignore Gibbon's spurious revisionist histories (he was a trenchantly anti-Catholic free mason) as well as Voltaire who wrote history to shape the political future, and reconsider Constantine in lieu of recent findings, at least regarding the sincerity of his conversion, and the importance of the Council of Nicaea in terms of eliminating gnostic elements as well as other heretical elements (e.g. those claiming Christ was not consubstantial with God; those claiming Christ was not fully human and fully God; those maintaining that the God of the Old Testament was a demi-urge and the God of the New Testament the ostensibly 'irrational' but loving supra-God). That Christianity was ever unified with a common orthodoxy (one tree with many limbs and branches), it was in the Roman Church, which cites Peter then James as first pontiffs. If you look at other denominations, they were largely driven by ego and passing corruption of the clergy by aristocrats (Lutheranism) and by political gambits (Anglicanism). There are a number of offshoots that were extremists of one sort or another (e.g. Marcionites who wanted to eliminate all Judaic elements in Christianity; Arians who believed in a unitary God and denied the divinity of Christ; and Anabaptists who were a motley of ascetics and lunatics).

What I would contend is that to the extent that there is legitimacy beyond Rome, to a lesser degree but still a considerable extent, it is to be found in Eastern Orthodox Christianity. Notwithstanding the theological confusion and corruption of the Nestorians, those Christian holdouts in northern Africa, which are almost all in communion now with Rome, are also legitimate, especially in terms of apostolic succession.

The Stark book: https://www.amazon.com/Triumph-Christianity-Movement-Largest-Religion/dp/0062007696

Rodney Stark isn't a Catholic, but his history "The Triumph of Christianity" is phenomenal. Even better sourced: Paul Johnson's "The History of Christianity". (Johnson is a Catholic, but he leaves no rock unturned and has no inhibitions about criticizing the Church). I'd say if neither of those two books convinces you, you'll remain convinced of your current position.

To the extent that this massive question and even greater answer can be summarized, I'd say that the Peterine and Pauline "Jesus Movement" (i.e. the Judaic and largely Hellenic groups) converged in large part as a result not just of the persecutions under Nero and Diocletian, but also because of the brutal Persian persecution of the church. I'd ignore Gibbon's spurious revisionist histories (he was a trenchantly anti-Catholic free mason) as well as Voltaire who wrote history to shape the political future, and reconsider Constantine in lieu of recent findings, at least regarding the sincerity of his conversion, and the importance of the Council of Nicaea in terms of eliminating gnostic elements as well as other heretical elements (e.g. those claiming Christ was not consubstantial with God; those claiming Christ was not fully human and fully God; those maintaining that the God of the Old Testament was a demi-urge and the God of the New Testament the ostensibly 'irrational' but loving supra-God). That Christianity was ever unified with a common orthodoxy (one tree with many limbs and branches), it was in the Roman Church, which cites Peter then James as first pontiffs. If you look at other denominations, they were largely driven by ego and passing corruption of the clergy by aristocrats (Lutheranism) and by political gambits (Anglicanism). There are a number of offshoots that were extremists of one sort or another (e.g. Marcionites who wanted to eliminate all Judaic elements in Christianity; Arians who believed in a unitary God and denied the divinity of Christ; and Anabaptists who were a motley of ascetics and lunatics).

What I would contend is that to the extent that there is legitimacy beyond Rome, to a lesser degree but still a considerable extent, it is to be found in Eastern Orthodox Christianity. Notwithstanding the theological confusion and corruption of the Nestorians, those Christian holdouts in northern Africa, which are almost all in communion now with Rome, are also legitimate, especially in terms of apostolic succession.