Having only perused Eisenman, I cannot say much about his evaluation of the Ebionite tradition that Paul was a Sadducee employed by the Temple police, tho I think this may have some merit, and it is possible that -- like Josephus -- Paul was a Hellenized Jew who tried out different Jewish faiths before settling on one. The main problem with viewing Paul as a Sadducee, as I see it, is that I see little trace of Epicurean and Sadducee thought in the epistles, or metaphors based on the priesthood (apart from Romans 15:16), whereas his total acceptance in the existence of angels, archons, principalities, resurrection, and especially his overall anthropology is not at all what one would expect from a latent Sadduceeism and definitely appropriate for either Pharisee or Essene thinking. On the other hand, the one text that is clearly Essene in character (in 2 Corinthians 6) -- as an isolated fragment -- has a good case of being a post-Pauline interpolation. Paul's employment of midrash is consistent with Essenism or Phariseeism but he is clearly an amateur in this matter, and so the Lukan depiction of Paul's synagogue training should be regarded as an unhistorical bit of pious hagiography. That would leave us with Philippians 3:5 and Galatians 1:14 which appear to be decisive, though it is always possible to argue that these texts have been reworked by a later Paulinist. I will probably defer to the most up-to-date criticism of these texts but it is not obvious to me what problem(s) these autobiographical comments pose. Paul's frequent obsession with the Law in his epistles, however, may be another clue. I don't think we're dealing with here a mind that has been totally Hellenized so as to think of Jewish-Christian concepts in a purely Greek light; Paul's anthropology, as mentioned before, is distinctively Semitic and does not evidence a sharp dualism that is found in fully Gnostic or Platonic (e.g. Alexandrine) texts, and from what I recall he similarly does not use phusis "nature" in Romans with the same Stoic meaning as can be found in 2 Peter and in philosophical treatises.
Anyway, regardless of what faith(s) Paul had as background, I think it does not change the basic point I was making -- that Paul could have started out some 14 years earlier with a less radical view on the Law and over time became fully antinomian. I see a clear three-stage process: (1) Gentiles were to become fully Law-observant and become circumcised, becoming in effect "Jews" and ceasing to be Gentiles. This is the traditional rabbinical view and I see this as the position of James, as presented in Galatians and in Ebionite tradition, and the "circumcision party" discussed in Galatians. (2) Gentiles were to become fully Law-observant but were not to become Jews, but rather Jewish and Gentile followers of Jesus were to form together a "new race," a "third race" neither Gentile nor Jew. This meant that circumcision and ritual separation was not necessary. I see this as the position of Peter/Cephas in Galatians and in Ebionite literature as well (echoed as well in 1 Peter). Thus Peter says in the Kerygmata Petrou: "He is a worshipper of God who does the will of God and observes the precepts of his Law. For in God's estimation he is not a Jew who called a Jew among men, nor is he a Gentile that is called a Gentile, but he who, believing in God, fulfills his Law and does his will, though he is not circumcised" (Rec. 5.43). I think this may have also been the position of the Didache, which does not specify circumcision among the requirements for Gentiles. The Matthean community also endorsed Torah observance and the Gentile mission but did not insist on circumcision. For some Jewish-Christians, the requirement for circumcision is spiritualized, and this is explicitly discussed in Gospel of Thomas 53:1-2: "His disciples said to him, 'Is circumcision beneficial or not?' He said to them, 'If it were beneficial, their father would beget them already circumcised from their mother. Rather, the true circumcision in spirit has become completely profitable' ". There is a close parallel to this in rabbinical Judaism; governor Turnus Rufus (the devil's advocate) is quoted as asking Rabbi Akiva: "If God takes such pleasure in circumcision, why then does not a child come circumcised from his mother's womb" (Tanhuma Tazria, 5). (3) Gentiles were not to become Law-observant and were not to become Jews, but rather Gentiles and Jews formed a new community whose "faith" releases them from sin rather than "works of the Law". The moderate position was perhaps a slippery slope for Paul; if ritual separation and circumcision are not to be practiced as works, perhaps other aspects of the Law are to be abandoned as well. The epistle of Barnabas spiritualizes the entirety of the Law, whereas Paul embraces the spiritualized understanding of circumcision that is also attested in the Gospel of Thomas (cf. Romans 2:25-29; 1 Corinthians 7:17-19; Galatians 6:15; Philippians 3:3; Colossians 2:11-14). Rather than spiritualizing the entirety of the Law for Christians, Paul simply says that it remains in force only for "Jews" who live as Jews whereas for Christians it is faith in Christ that justifies sin and releases mankind from the Law. Paul could have had this radical position from the start, but considering his "neither Jew nor Greek" statements and adoption of the spiritualized understanding of circumcision, I think it is quite likely that at one time his position was closer to (2) than (3).