Yes but when those religions were "established" they had a top-down model, the religious elite was also the social elite of the respective political entity... Christianity was on the other hand a bottom up thingy the membership consisted (mostly) of the powerless part of the Roman society. Thus they needed a solution, which allowed peaceful coexistence with the Roman authority, set the bar low enough to attract enough people, yet made them distincitive enough.
Salvation through faith (and only faith) is an excellent solution for the problem
That implies a conscious will by Paul's to start a sort of cult with shady intentions. I've read theories defending that position, and I find them quite unconvincing. In my opinion, what emerges from the lecture of Paul's undisputed epistles is a man who was sincere in his beliefs, as they were based in some sort of deeply personal, mysthical experiences ("visions") that for Paul had been entirely real. He attributed them to the (by then) dead Jesus of Nazareth, and became convinced that as he'd been the only man who the Risen Christ had talked directly to after his "earthly" death in his divine epiphany, that his version of Jesus' message was "superior" to the one known by those who had known Jesus in the flesh, during his "earthly" life (basically the original apostles and their followers, by then still centered in Jerusalem and led by Peter/Cephas and James the Just).
His epistles reflect Paul's somewhat unsuccessful attempt at bridging the chasm between his understanding of Christ's message as "revealed" to him in his visions (deeply innovative and which marked a radical break from Judaism) and the earthly Jesus' preachings as still recalled and recorded by the Jerusalem community, who considered Jesus and themselves still as Jews, and as bound to the observation of the Law as understood by the Pharisaic tradition (with an oral Torah which complemented the written Torah of the Scriptures), and to the Final Judgement as preached by Pharisaic and Essene rabbis.
The extension of God's grace and the possibly of salvation from eternal damnation in the Last Judgement, which Pharisees and Essenes believed was restricted strictly to Jews, was extended by Paul to all mankind, but probably not as part of a cynical marketing plot, but because Paul, having been born in a cosmopolitan Greek.speaking town in Cilicia and having lived all his life among gentiles and knowing Greek culture and philosophy, was a man of less provincial leanings than the original apostles (and Jesus) who, for all we know, had never left Palestine and were only interested in Jewish culture and religion. Contrary to them, Paul probably felt more at home in Ephesus, Antiochia or Corinth than in Jerusalem.
