The proposition assumes that ALL early Christians thought the same, believed the same.
If you care to take the time to research first century Christianity, you'll find first of all that in the beginning at least the Jerusalem congregation could still consider themselves as part of Judaism, bearing in mind that Judaism itself was divided into factions. And secondly, that there was not neccessarily any unity of thought among the different groups of Christians.
One of the reasons Constantine had in mind when he called the general council of the church in Nicaea was to attempt to unify the church.
So to argue, that some group today is or is not Christian, you must first define true Christianity in the first centuy