Remnants of animal worship survived in Judaism and Christianity. Satan was a serpent; Jehovah, like Osiris, was worshiped as a bull; Christ was the lamb of God, and the Holy Ghost appeared in the form of a dove. However, these are only symbols of the worshiped object, and are not worshiped in themselves.
The Egyptian religion can also be considered the oldest religion. Its origins date back beyond 3000 BCE.
Religion comes from "religare," which is to bind, or have union with, etc. so humans throughout our history have been seeking this union. Yoga, in Sanskrit, means union. This longing for connection to god, to worship god, is innate in most all human beings.
Hinduism: Since Sanskrit is the oldest written language, as far as religion being written down, and as far as organized religion is concerned, it would be Hinduism. It is said that when Rama appeared, according to our calendar, was a million or so years ago (not sure of the exact date, but it's a long time!) And Krishna, god himself according to the Vedic scriptures, appeared here 5,000 years ago. Buddha, about 500 B.C., Jesus, about 2,000 years ago. If you go through the different religious book and study this question deeply, you will find out that Hinduism is the oldest religion of the world. There are no dates and facts, but its history is about more than 50000 thousands years ago....
Further reading...