Worshiping multiple gods does not mean allowing the worship of all gods. Ancient Egyptian pharaohs would often declare one particular god to be the chief god during the pharaoh's reign, and woe betide any who did not bow down to that god. I'm not familiar with the Oriental religions, so I can't comment there, but I know that the Romans were considered unusual in that they did not force their conquered subjects to convert to the Roman religion. Once the Catholic Church took over most of Europe through the Holy Roman Empire, the Inquisition tried to make short work of the worship of any other gods. We can all see how intolerant Islam has been shown to be, and at least here in the US, the religious right has become quite vocal, and occasionally physical, about suppressing non-Christians, especially Muslims and atheists.
It's been a relatively common thread throughout the history of Western civilization, and I believe in the Aztec, Maya and Inca cultures as well.
As for which gods are oldest, a study of religion will show that virtually all religions trace their roots back to the very first historical references of gods. The names may change, and the details get blurred and mixed, but the tracks are there to be followed. Neither Christianity nor Islam, for example, would exist without the solid background of Judaism, and Judaism itself was preceded by older religions, which I'm sure were derived from still older religions. All of the gods trace back to the very dawn of history, and probably beyond.
And as for the Catholic Church, I don't want it to be evil. But modern communications and education have shown that it is indeed far more evil than anyone ever knew. History has shown that the Vatican has been a barrier to human advancement throughout its history, using its power to control and suppress large portions of the world's populations, breeding poverty and misery while claiming to fight against them.
The greatest advantage of modern communications, especially the internet, is that the kinds of atrocities the Vatican condones and even commits can no longer be kept hidden from the world. Within minutes of something happening reports are spread far and wide, disseminated irretrievably, exposing government and religious leaders for the cowardly hypocrites they truly are.
But you are right, denu: the Catholic Church is not as evil as I thought it was. It's far, far worse.