When I was a kid, my father always told me not to tell lies because if you do, you need a very good memory whereas if you tell the truth, you don't need to remember what lies you told before.

Now when these ancient people went around preaching their nonsense, no-one was writing it down and they said one thing one day and something different another day. They'd forgotten what they'd said previously or chose to say something different to a different audience.

When things were repeated and repeated (parrot fashion?) until many hundreds of years later, one group remembered what had been said to them and wrote it down and a different group wrote down what they'd been told. When it was all put together, you then find differences and complete contradictions showing that really, it's all a complete pack of lies.