By far Hindu is the oldest currently practiced religion, it goes back to pre-written history in the Indus valley. However there are some older nature religions that are practiced that may be just as old.
Christianity was NOT the first religion! how ignorant do you have to believe that nonsense. The very first written account of religion was the Mesopotamian mythology, believed by the Sumerians. According to Mesopotamian mythology, the gods, called the Annunaki, originally created humans as servants for themselves, but freed them when they became too much to handle. The Annunaki were believed to be from the planet they called Nirubu, which has a long elliptical orbit, and is actually part of our own solar system. interestingly, scientist have recently discovered a planet, nick named "planetX" that matched the orbit described by the Sumerians over 5000 years ago. Its orbit is so long that it is only visible every 26,000 years or so.
The oldest practiced might be Hinduism or Kemetic. Kemetic has come back recently. It was started by Pan-Africanism. Kemet is ancient Egypt. So we follow the old Egyptian ways. Another form is Atenism, which is monotheistic. Considering the similarities, the founders of the Jewish faith might have originally been Atenist. Also Druidism is back, and that is old.