Well, one would say that merely being ‘the oldest’ religion does not make something the truth.
The oldest theory about the formation of the universe was that of its formation by some god or goddess. Does that make it the truth because it was first?
When you are a child, you understand things in a different way from when you are an adult. It is not surprising that the rudiments of Christianity (or Judaism or any other religion) are all found in those religions.
Christ did not appear to CroMagnons, he appeared in the first century AD.
The Jewish religion had to understand the idea of gods before it could understand ‘one God’.
So I have no trouble at all understanding Catholicism as true. Nor do I have difficulty understanding that parts of the faith are believed and were believed before Jesus walked on this earth. Why wouldn’t there be grains of truth in all religions if all are seeking God? And why wouldn’t God when ‘the fullness of time’ suggested people finally could give a decent understanding OF the full truth, send His Son to give us that truth? And why wouldn’t people who had already glimpsed it in other religions but were hungering for more be ready for it? Conversely, why wouldn’t people ‘fight back’ by clinging to the old, or reject this ‘Christianity’, because they thought they already ‘had sufficient’ or didn’t want to have any truck with some god anyway.