The earliest texts of the New Testament (before the Gospels) are considered to be by Paul - certainly the first Epistle to the Thessalonians, and possibly even earlier, Galatians. This is the late '40s, early '50s. Jesus is held to have died in 33 (give or take a few years). The set of beliefs around JC's divinity and sacrifice for the sins of mankind grew very fast. Paul had a major hand in shaping them.

There is neither Jew nor Greek, there is neither bond nor free, there is neither male nor female: for ye are all one in Christ Jesus. (Galatians 3:28)

This is one of the earliest statements of Christian doctrine (again, written before the Gospels), and it's a declaration of the universality of Christianity. It goes considerably further than the Jew/Gentile question, since it includes slave/free and man/woman. I don't know whether Paul came up with it just because he wanted to evangelize Gentiles, or because he had a sharp eye on the marketing - but quite possibly he was consumed with passion for the revolutionary belief expressed there, and he was not alone in holding it, as early as AD 50.

My point being that the post-hoc rationalisation idea doesn't correspond to history; the earliest known statements of Christian doctrine are in fact universalist.