Then who did? There's the Old Constantine canard. This is odd, since if Constantine was so involved in the First Council of Nicaea, should it have reflected Constantine's Theology? Constantine was (at least at the time) more sympathetic to the Arian beliefs (Arians believed there was a "time when the Son was not"). If Constantine was so involved, why is modern Christology so Trinitarian? And then you hear the argument that the Church was influenced by Paganism, then it would have had to be so early on. Consider those who claim that the Eucharist is a Pagan invention. Well then, I guess St. Paul (1 Corinthians 11:23-30) was a Pagan! What of St. Ignatios of Antioch (one of my favourite Apostolic fathers), if he didn't believe that this was really the Body and Blood of Christ, why did he write so vehemently defending the Eucharist? Furthermore, what Pagan religion believes that Bread and Wine become their god or gods? Egyptians, Mithraic cults, Roman and Greek Pagans, had meals of bread and wine offered to their gods, but they did not, believe that they became the Body and Blood of said gods. (Side note: It actually seems, that Mithraic cults borrowed from Christianity, not the other way around, Justin Martyr had some writing on this issue). If Christ did not found the Catholic Church, than those that attack the Church as being founded by Constantine may have a point. If it wasn't for this. Most people who call themselves Christian accept the canon of the New Testament. Which was given to them by the Church. (For the record, I am a supported of the earlier dates of the New Testament books), most would accept the Doctrine of the Trinity, no problem, the Virgin Birth, and the Sinless of the Lord, and many also accept the Apostle's Creed. The Anti-Catholic's own logic, would mean, that if their assertion were true, their beliefs would be from Constantine, not from God. Good thing we Catholics have history on our side. Just remember, what the Lord said to Simon Peter about the Church "And the Gates of Hell shall not prevail". And they most certainly have not!
-Adam Charles Hovey