It's tricky. The Catholic Church claims to be the original Christian religion, with the idea that the Popes form a chain of leaders that go all the way back to Peter the apostle as the first leader of the Church.

But it all depends on your perspective. In reality (or at least my opinion), it's more of a family tree of sects branching off from each other that all go back to the original followers of Christ. Of course, ancient Christianity would have looked very different from all modern religions, so it's a bold claim for any to say it is the same as that original group.

From a Protestant perspective, Catholicism was far from being the first Christ-based religion. In fact the whole reformation fiasco that took place in the ~1400-1600s was a reactionary movement that sought to return to older, original representations of the faith as portrayed in the time of the New Testament’s writing. This movement was fueled by a critical thinking process that spurred a return to the original texts of the religions, and a realization that Catholicism had much in both its rituals and its canon that was nowhere spoken of in the original ethos of the faith.

>but would I be incorrect saying Catholicism was the first Christ based religion?

Absolutely.

The first Christians faiths were the original cults that sprang up in the wake of Christ's death circa 30ish AD.

Those cults then spread across the Eastern Mediterranean and started forming their own religions, and were only broadly unified into a single church a few hundred years later by the Council of Nicaea in 325 AD, which is broadly considered the origin of the original Christian "Church" as accepted by Roman Catholicism, the various Orthodox Churches, the Assyrian Church, and the Anglicans, and (as a result) by Protestantism further down the line.

The Catholic Church as it exists today was only really formed 700 years later, when the Greek Orthodox and Roman Catholic churches split from each other in the Great Schism. But even prior to the Schism there were tons of variant churches all following the Nicene Creed; the only reason the pre-schism Catholic Church was so large was because of it's relationship with the Roman Empire.

Honestly; if you're looking for the faiths closest to OG Christianity, that would be either the Coptic, Oriental Orthodox, or Assyrian Churches.