But the doctrine of the Sadducees is this; that souls die with the bodies. Nor do they regard the observation of anything besides what the law enjoins them … For when they become magistrates; as they are unwillingly and by force sometimes obliged to be; they addict themselves to the notions of the Pharisees: because the multitude would not otherwise bear them. (Josephus, Antiquities,18)
And when I was about sixteen years old, I had a mind to make trim of the several sects that were among us. These sects are three: - The first is that of the Pharisees, the second that Sadducees, and the third that of the Essenes, as we have frequently told you; for I thought that by this means I might choose the best … So when I had accomplished my desires, I returned back to the city, being now nineteen years old, and began to conduct myself according to the rules of the sect of the Pharisees … (Josephus, Life)
And when I was about sixteen years old, I had a mind to make trim of the several sects that were among us. These sects are three: - The first is that of the Pharisees, the second that Sadducees, and the third that of the Essenes, as we have frequently told you; for I thought that by this means I might choose the best … So when I had accomplished my desires, I returned back to the city, being now nineteen years old, and began to conduct myself according to the rules of the sect of the Pharisees … (Josephus, Life)
When Josephus claims to have returned to the city and engage in city affairs, it would be entirely logical for him to have adopted the precepts of the Pharisees as those were apparently the precepts under which the preponderance of Jewish civil and legal affairs were conducted at the time.
I am not aware of any compelling evidence in Paul’s letters to take Paul’s claim in Philippians any further than he had followed the precepts of the Pharisees when involved in Jewish civil affairs. Such was apparently the norm in his day in Judea, and likely also in Jewish enclaves in the larger Levant cities like Damascus.
There is no clear evidence in Paul’s letters for the existence of Jewish Christian missionaries working outside of Judea in Paul’s day. Not in Galatians, not in Philippians, and not even for the so-called ‘super-apostles’ in 2 Corinthians. In contrast, Jews were reasonably widely distributed in the areas of Paul’s work, including in Asia Minor.
It’s not hard to see how local Jews would object to Paul’s converts claiming to be full-participants with the Israel of God without the clearly required ritual of circumcision. Conjuring up Jewish Christian opponents hassling Paul’s converts does not find adequate support in the letters, and seems to be based on apologetic translations in the case of 2 Corinthians. The ‘super apostles’ could very reasonably have been professional Jewish missionaries working the Diaspora circuit. Dieter Georgi makes a compelling case for the activities of such Jewish missionaries in chapter 2 of his, The Opponents of Paul in Second Corinthians.
Taking this even further, is there any evidence clearly independent of Paul for the existence of predecessors in the faith in the form of Judean assemblies in Christ, or the leadership triumvirate in Jerusalem? I would actually be thrilled to see it, but I’m not currently aware that any such clearly independent evidence exists.
Absent such independent evidence, I think the most likely solution is that Paul contrived those Judean predecessors to provide a sense of tradition, a spiritual movement in the far away Judean homelands to provide a foundation for his evangelizing work. And Paul trotted-out those figures in Galatians and 1 Corinthians to provide support for his positions and arguments.
The convention of a Judean origin for the Christian faith is so strongly established that I see little hope for any significant movement on that issue for a very long time --- despite the complete lack of any evidence clearly independent of the questionable claims of a single entrepreneurial Jewish specialist on the hunt for Gentile patrons far removed from the Jewish homelands.
ETA: Assumptions here (that I’ve accepted based on my own studies) --- Galatians, 1 and 2 Corinthians, Philippians, and 1 Thessalonians are authentic and adequately intact, and GMark is dependent on Paul's letters. If one doesn't accept those assumptions --- fine and dandy.