It’s a lot easier to hijack something than to fabricate it. Instead of inventing a totally non-existent character of Jesus Christ and selling that product to the masses, it makes more sense that the authorities would consolidate already existing paradigms into a single system. The purpose of such a system would be to portray these authorities as divinely sanctioned.
Assimilated paradigms included Mithraism, Greek Neo-Platonism, and Judaism. For instance, the solar/zodiacal elements in Christianity came from Mithraism and related pagan systems. But these are just auxiliary additions, not the core nucleus of Christianity. The nucleus is the original Christian system, which existed alongside the other elements prior to their combined assimilation into an organized religion.
All of these elements were then glued together by the authors of the New Testament, who added various rationalizations, logical sleights of hand, fabricated backstory, and gaudy superstitious elements to create a belief system that would appeal to the widest audience. The Gospels of Mark, Matthew, Luke, and John were variations of the same story further customized for Greek, Jewish, and Roman audiences.
Thus the New Testament is a patchwork of truth and lies. The original pre-assimilation Christianity is buried in there somewhere. How can it be found? By stripping away the rationalizations, sleights of hand, superstitious miracles, pagan symbolism, etc… This is done the same way one approaches any potential source of disinformation; you look for logical fallacies and attempts at persuasion that would play into a negative agenda