Nearly any scholarly look at the history of Christianity will show that it started out as a sect of Judaism. I know until the 4th century at least, there were communities of that sect that still was in existence. I don't think they went that further than that.
There was also Pauline Christianity; however, that was still a Jewish sect, or considered to be so at that time. The Catholic Church claims to be based from that, but there is no credible evidence to really support that. Pauline Christianity evolved into something else, but it does not exist today. It loosely evolved into something that did not resemble Pauline Christianity at all.
During that same time, there also became some Gnostic sects which competed with Paul to a point, and more so with later Christians (especially in the second century). They were later wiped out over time. Mainly, what would become the Catholic church gained more power, and influence. During that same time though, there were a handful of other Christian sects as well. The ones that jump into my head are the Markans and Ebionittes (the spelling may be off on those).
Basically, early Christianity was extremely diverse. It was even more diverse than it is today. However, it can be known that the earliest form was a sect of Judaism.
As for the oldest surviving group, the Catholics would be that.