Vedas are the oldest piece of knowledge known to human kind. They are perhaps the source of knowledge contained in all other religious scriptures.
Pundit Ganga Prasad Rai Bahadur have made a detailed comparative study of the principal religions of the world and a manifestation of common origin from the Veda. His research is published in the following book:-
Title: Fountain Head of Religion
Author: Rai Bahadur, Pundit Ganga Prasad
Publisher: Shri Ghoodmal Prahalad Kumar Arya Dharmartha Nyas
Byaniya Pada, Hindaun City, Rajasthan - 322 230, India
Ph: +91 7469 234624
Also available at www.vedicbooks.com
The author in his theory, traces the Vedas back to being the oldest written source of theological systems. He takes major religious themes like good and evil, the after life, resurrection, and the name used for the God in the religions of Judaism, Christianity and Buddhism (among others) and traces them confidently back to the Vedas. He also concludes that all religions are traceable to Vedas and the Vedas are not traceable to any older religion.