>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.