Regarding the charge that Paul was a turncoat, abandoning “true” Judaism, we must first understand a number of points.
First of all, as any decent scholar will tell you, Second Temple Judaism involved a number of different “factions”, and for convenience we use the terms found in the NT, Josephus, and the Talmud: Pharisees, Sadducees, Essenes (“Zealot” and “Herodian” also, but these arguably represent a different vertex in our construction). Now, there were some core beliefs held by all groups (e.g. monotheism, Israel as the covenant people of God, morality is important, etc.), but there was a lot that was different between them (e.g. resurrection, angels and spirits, ritual and moral purity of the Temple, priests, and Levites, approach towards Rome and Gentile dominion generally, etc.).
Secondly, although Christianity was persecuted in the land of Israel, there were also times and occasions when Christian and non-Christian Jews were able to live peacefully together and even attend synagogue together. Much of the Early Church was ethnically and culturally Jewish, still worshipping in the Temple before its destruction. Many would have had no problem in saying that Christianity was part of Judaism, even if they didn’t share its tenets. Even the great Gamaliel was willing to withhold judgement on the matter (Acts 5:33-35).
Now obviously Christians and Messianic Jews disagree with Jews who don’t believe that Jesus is the Messiah. The ruling on this question answers the question of whether Paul found or abandoned the true faith. But we can usefully ask, “Did Paul have good reasons for his choice?” We cannot know the exact nature of Paul’s conscience in his pre-conversion life, but Jesus’ words to him, “It is hard for you to kick against the goads”, suggest he was feeling something. And well he should have: for the Christians he was persecuting were performing good deeds: they worshipped at the Temple, they cared for each other spiritually and materially, and they healed the sick (Acts 2ff).
Now we come to his vision of Jesus on the Damascus Road. If we take an extremely sceptical view, we can write it off as a hallucination produced by his subconscious mind to vent the guilt he was feeling for the terrible things he was doing. But this doesn’t really account for his conversion and Apostolic ministry. All he needed to do to assuage his guilt was stop persecuting them and make amends. Preaching the Gospel would not fall under this category.
Paul actually believed that the Gospel message was true, and to understand this, we need to look at his exegesis and doctrine and passages in the Tanakh. The true Gospel is unique when we set it against other views of the Saviour and the Eschaton. Paul’s Gospel proclaimed Jesus of Nazareth the Messiah, crucified and risen, to take away the sins of the world. All who believed on Him would be saved, and He would come on the Last Day to receive His Bride unto Himself and to judge the World. Salvation was to be found in no one else, our own works would not help us. The Tanakh, according to Paul, testified of all this.
What was Paul’s evidence for these points? On some points we have his own words, on others we will have to conjecture, assuming he agreed with the Gospel writers (which seems likely given the fact he knew Luke and had contacts throughout the Early Church – he gave evidence before the gathered Apostles and elders at the Jerusalem Council). Some traditions also hold Paul to be the author of the Epistle to the Hebrews (personally I agree with this assessment for various reasons): this letter also furnishes us with Christological and soteriological exegesis.