We cannot talk about early Christianity in the singular because it consisted of diverse groups. Given this diversity, we must speak of multiple Christianities.
We cannot talk about parliament, whether British or Canadian or Australian, in the singular, because it consists of diverse parties. Given this diversity, we must speak of multiple British parliaments, or multiple Canadian parliaments, or multiple Australian parliaments.
We cannot talk about any university in the singular, because any university consists of multiple faculties. Given this diversity, we must speak of any university as multiple universities.
We cannot talk about a given human body in the singular because it consisted of diverse organs. Given this diversity, we must speak of any given human body as multiple bodies.
These propositions are all horribly flawed, and they are flawed because they ignore the concept of level. A body is not an organ, but rather a higher-level abstraction in which organs exist. A university is not a faculty, but rather is a higher-level abstraction in which faculties exist. A parliament is not a party, but rather is a higher-level abstraction in which parties exist. Early Christianity is not one of the diverse groups that made up early Christianity, but rather a higher-level abstraction in which those groups existed.
Of course, it might be objected that, unlike parliament, university, or the human there was no formal unity that constituted a higher-level of abstraction in which diverse Christian groups operated. That is a hypothesis, and one that needs to be substantiated. The problem is that the data is hostile to that hypothesis. If we use "formal" in the colloquial sense of "official" then perhaps it might hold, in that early Christianity was by definition "unofficial" until the 4th century. But then again, so too would it hold for the human body. If we use "formal" in the more technical sense of form, then the evidence is really quite strong for a formal field in which early Christian groups operated. It took the form largely of movements and communications between congregations existent in the various cities of the empire, and perhaps at an early time points east. These lines of communication and movement do not obviate but in fact were primary vectors for conflict. Conflicts in Jerusalem spilled over to Antioch spilled over to Galatia, for instance. Of course, in the first couple generations this formal, unofficial unity was perhaps relatively inchoate, as novel phenomena often tend to be, but it still existed.
There was an early Christianity. It was singular. It was pluriform. These statements are not oxymoronic, not if one understands the difference between parliament and party, university and faculty, body and organ.