Despite being clearly written and a mere 143 pages long, “St. Paul” is an often dense and exasperating book. Karen Armstrong, a popular and prolific authority on religion (“A History of God,” “The Battle for God: Fundamentalism in Judaism, Christianity and Islam” and, most recently, “Fields of Blood: Religion and the History of Violence”), wants to rescue Saint Paul from the reputation he has acquired as an authoritarian and misogynist. According to her, such accusations are the result of misreadings or tampering by later, less egalitarian-minded editors with Paul’s “authentic” New Testament writings. Instead of the often oblique and even inscrutable Paul we find in scripture, Armstrong’s apostle is a kind of gloried community activist, or a first-century Bernie Sanders. Paul, in this reading, evolved from a fervent proselytizer for the risen Christ into an “intrepid opponent of empire” whose religious convictions were “less about doctrine than a social imperative.”