It is, as Newman commented long ago, plain enough that there are differences between the Church as we see it when it is legalised in the Roman Empire in the fourth century, and what is described in the Acts of the Apostles. This occasions no surprise to any historically-aware person; across that range of time and cultures it would have been astonishing if things had not changed. Some changes were relatively painless but undocumented – no one insisted that every successor to the Apostles should be a Jew, although there was a consensus that elders/bishops should be male; others were painful and documented – Gentiles did not, as it turned out, have to be circumcised or obey kosher laws. The transfer of the sabbath from the Jewish Saturday to Sunday was an especially painful change, as there were many who insisted that it must remain where the Apostles had it; but the Church decided otherwise. However, for many years, there were those who refused to accept this and who alleged it showed that the Church had turned away from the right path; no doubt the ‘men from James’ felt the same. In short, there has always been change and always been those who objected to it.
Nor was this change confined to what one might call the ‘accidents’ of the faith. In many posts here on the Trinity (just enter the term in the search panel and you will find more than I can list here) we have seen that the attempt to understand what St John meant by writing that the ‘Word’ was ‘in the beginning with God’ and ‘was God’ took pious and intelligent Christians in different directions. The Arians thought that because Jesus was the Son, and the Son came after the Father, that Jesus was the first-born of all creation. When Arius formulated that view, supported by many Scriptural quotations, it was not heretical because the Church had not actually come to a fixed view; in fact it was Arius’ teaching of a view which clearly made Jesus ‘a creature’ which forced other Christians to think very clearly about the issue.
This is brought out well in one of my favourite books by Newman, Arians of the Fourth Century. Now, had Arius not been free to enunciate his views, no doubt a good deal of pain would have been averted, but the result of his boldness was to make Athanasius and the Cappadocian Fathers think carefully about why Arius was wrong. Their long argument with Arianism and its variants were the whetstone on which the doctrine of the Trinity was sharpened. Those who still refuse to accept the Trinity rarely show any acquaintanceship with the Cappadocian Fathers or with Athanasius, and, indeed, some do not know that they are Arians and that their arguments were exploded long ago. To lack a sense of history is to risk repeating what was once not heresy, but is now and will be for ever more.
However, for there to be heresy, there has to be an authority which declare orthodoxy, and for Catholics, this is what Jesus provided for when he gave the powers of binding and loosing to St Peter and his successors. For Orthodox Christians, this power lies with the successors of the Apostles and the Councils, a position which the Catholic Church regards as not going quite far enough as it does not recognise the special place of Rome. But without some authority, who is to say what changes, what developments are orthodox and which are not?