He says, in addition, that “all the Christians were of one mind,” not observing, even in this particular, that from the beginning there were differences of opinion among believers regarding the meaning of the books held to be divine. At all events, while the apostles were still preaching, and while eye-witnesses of (the works of) Jesus were still teaching His doctrine, there was no small discussion among the converts from Judaism regarding Gentile believers, on the point whether they ought to observe Jewish customs, or should reject the burden of clean and unclean meats, as not being obligatory on those who had abandoned their ancestral Gentile customs, and had become believers in Jesus.
Nay, even in the Epistles of Paul, who was contemporary with those who had seen Jesus, certain particulars are found mentioned as having been the subject of dispute—viz., respecting the resurrection, and whether it were already past, and the day of the Lord, whether it were near at hand or not. Nay, the very exhortation to “avoid profane and vain babblings, and oppositions of science falsely so called: which some professing, have erred concerning the faith,” is enough to show that from the very beginning, when, as Celsus imagines, believers were few in number, there were certain doctrines interpreted in different ways.
Source: Against Celsus (New Advent)