The problem with this half-truth is that the language-game for being Jewish or Judaism has nothing to do with a particular religion or ideology. In Jesus's day you could adopt one of several explanatory models of what Jewish faith was about (Essene as in the Dead Sea Scrolls, Pharisee, Sadducee, Zealot). In our day Jews can function as Chasidim (as in the books of Chaim Potok), Atheists, Orthodox, or Zionists, and even Marxists (Karl Marx remained a Jew).
Paul was certainly attacked by an unholy alliance of Pharisees and Sadducees who agreed that he deserved to die. And he was crucified for denying making statements which did not fit their views. But many were martyred in England for similar reasons without denying that they were Englishmen. So it is true that Paul did not cease to be a Jew (Acts 22:3, Romans 11:1) when he began to preach our Trinitarian God of grace, including the power of the Holy Spirit to do in us all that legalism had failed to deliver.
Sadly by the second century it was assumed that you could not be both a Jew and a Christian. Conversion and denial of one's Jewish heritage was required. This became an essential dogma of the mediaeval church and of all later evangelical denominations. In the nineteenth century it caused tragic disruption in hundreds of families such as the Disraelis, Mendelssohns, and Wittgensteins. And it was was one of the main causes of the holocaust.
The Messianic Jews have got it right, and there are reports that their congregations are growing rapidly in Israel. Being a Jew is a matter of race, and you can believe in Jesus as the Messiah without losing any of your Jewishness. What happens if in the next few years Christian Jews became one tenth of the citizens of Israel in the same way as Christians in Canada? And how would the relationship between Christianity and Judaism look if large numbers of Messianic Jewish synagogues in North America were counted as part of Judaism?
Schowalter has also missed the fact that the only definition of a Christian
given in the New Testament is in the Book of Acts. "It was in Antioch that
the disciples were first called Christians" (Acts 11:26). By that
definition to deny that Paul was a Christian is nonsense.