From a Christian theological perspective, one would say that God quite obviously chose St Paul for this Mission to the Gentiles. Ergo, my statement applies perfectly well from a Christian standpoint: It was St Paul who transformed an obscure Jewish messianic sect into a universal creed because God chose him to do so. From the perspective of the historian, it is irrelevant that God could've just as well chosen someone else or that Jesus would've prevailed in any event, as a Christian would say, because in the actual course of history it was not someone else, it was St Paul.
So, from the historian's perspective and from the theologian's perspective St Paul was the most influential man ever to have lived, because without the influence of St Paul there would be no influence of Jesus Christ, even if in the absence of St Paul someone else would've achieved the Mission to the Gentiles. No one else tried though, according to the New Testament. In fact, they opposed Paul's Mission and wanted to preserve the Jewish law. Had the Jewish law been preserved, Christianity wouldn't even exist, because it would then a priori be merely one Jewish sect among many, all of which were stamped out with the rise of rabbinic Judaism.
And might I add that the Greco-Roman world rightly regarded circumcision as a perverse barbarity. If nothing else, it was Paul's steadfast rejection of circumcision that made it possible for Christianity to spread amongst the Gentiles. It was not until the Victorians went nuts that circumcision was reinflicted on the Christian world.
But, there was a whole lot of "else" to Paul's mission as well. From a historical standpoint, St Paul is absolutely indispensable to the emergence and triumph of Christendom.