The Buddha said that the Dhamma which he preached was unheard of before, but was clear to his vision, to his knowledge, to his wisdom, to his penetration, and to his enlightenment.
Some people claim that the Buddha did not preach a new doctrine but merely reformed the old teaching which was existing it that time in India. However, the Buddha was no mere reformer of Hinduism as some protagonists of this ancient creed make him out to be. The Buddha's way of life was substantially different than the way of life into which he was born.
In the history of ancient India, the Buddha lived, taught and died as a non-Vedic, non-Brahmanic and non-theistic 'teacher of gods and men.' Nowhere did the Buddha acknowledge his indebtedness to the Upanishads. The Buddha considered himself as initiating a new method, as opening a new path.
The Buddha was regularly condemned, criticized and insulted by the most noted teachers and texts of the Vedic-Brahmanic tradition. It was with the intention of destroying or absorbing the Buddha and his teaching, that the Brahmans of the pre-Christian era went so far as to accept the Buddha as an Avatara of God. Yet the Buddha had been despised as a vasalaka, a mundaka, asamanaka, a nastika and sudra. (These words were used in India during the Buddha's time to insult a religious man).
There is no doubt that the Buddha reformed certain customs, religious duties, rites and ethics and way of living. The greatness of his character was like a pin-point that pricked the balloon of false beliefs and practices so that they could burst and reveal their emptiness.
But as far as the fundamental, philosophical and psychological teachings are concerned, it is groundless to say that the Buddha had copied ideas from any existing religion at that time. For instance, the idea of the four noble truths, the eightfold path and Nibbana, were not known before his coming. Although the belief in kamma and rebirth was very common, the Buddha gave quite a logical and reasonable explanation to these beliefs.
Taken from "What Buddhists Believe"
Written by Ven. K. Sri Dhammananda
Published by Buddhist Missionary Society