Zarathustra aka Zoroaster aka Zartosht (probably lived 6th century BC, Persia)

Truth is best (of all that is) good. As desired, what is being desired is truth for him who (represents) the best truth. (Gathas 27.14)

Doing good to others is not a duty. It is a joy, for it increases your own health and happiness.

With an open mind, seek and listen to all the highest ideals. Consider the most enlightened thoughts. Then choose your path, person by person, each for oneself.

If one would have a friend, then must one also be willing to wage war for him: and in order to wage war, one must be capable of being an enemy.

Turn yourself not away from three best things: Good Thought, Good Word, and Good Deed.
(quoted from the Gathas, which are a part of the Avesta)

Zarathustra
"Zarathustra on the Mountain" by Nicholas Roerich

The Iranian prophet Zarathustra (c. 1400 B.C.E.) - also known by the Greek form of his name, Zoroaster - the oldest known prophetic founder of a religion. Although the Zoroastrians’ own tradition states that Zoroaster lived 258 years before Alexander the Great (c. 600 B.C.E.), this is a late calculation based on a Greek fiction. Other Greeks argued that Zoroaster lived 6,000 years before Plato, or 5,000 years before the Trojan War, emphasizing his antiquity. Scholars now argue on the linguistic and cultural evidence of the Gathas (Hymns), the seventeen hymns written by Zoroaster that form part of Zoroastrian scripture (the Avesta [The injunction]), that the prophet lived sometime between 1500 and 1200 BCE, most likely in about 1400. This dating is consistent with the very early form of language in the Gathas, which is similar to the Indian Rig Veda, and also with the pastoralist, preagricultural life Zoroaster’s people are described as living.
Zoroaster may have lived in what is now Kazakhstan, but his people finally settled in eastern Iran. The religion he taught had a world-reaching importance as the belief of the rulers of the Persian Empire. Zoroaster was a priest of the traditional religion, apparently a poor man. He seems to have lived at a time when warfare was on the rise and traditional values were breaking down thanks to contact with the more settled peoples of the west and south. Certainly the Gathas include a passionate sense of the powerlessness of the poor. Even more movingly, an entire Gatha is devoted to the plight of the defenseless cow, unable to protect itself in a world of increasing violence and disruption. This unsettled world was the background to a series of visions in which the supreme god, Ahura Mazda (Lord Wisdom) appeared to Zoroaster, calling him a prophet. Zoroaster set out to reform the traditional religion. He did not deny his polytheistic heritage but taught that in the beginning there was only one god, Ahura Mazda, who was creator and upholder of asha, the principle of truth and order in the universe. But this good god found himself opposed by an antagonistic principle, drug (deceit, or “the lie”), given form as the spirit of destruction, Angra Mainyu. They began a cosmic struggle, one that will eventually end in the victory of good and order. Each force created subsidiary gods to help in this ongoing battle, and the world itself was created as a battlefield between order and chaos.
Zoroaster can fairly be said to be the first proponent of a religion based primarily on ethics rather than sacrifice. He taught that humans are absolutely central to this battle of good and evil—they must choose to support either Ahura Mazda or Angra Mainyu and must live their lives in a way to aid the battle for good. Thus morality is central, although Zoroaster did not reject the role of ritual in religion. In the end, good will triumph, in a great “Making Wonderful” that will include punishment for evil and reward for good—an idea adopted by other religions, including Christianity. The traditional priests did not appreciate Zoroaster’s efforts at reform, and he was driven away from his native land. He found refuge with a prince named Vishtaspa, whom he converted to the new religion. Settling at court, Zoroaster preached his prophetic vision to rich and poor, men and women. With royal patronage, he became wealthy and lived a long life, perhaps reaching the age of seventy-seven. His teaching was given validation when, egged on by traditional priests, neighboring states attacked, only to be defeated by Zoroaster’s supporters. Finally, according to Zoroastrian legend, he met a martyr’s death, killed while carrying out his priestly function at the fire altar, either by a rival priest or in a raid by a neighboring tribe.
Later Zoroastrian legend emphasized the prophet’s role as an emissary sent by Ahura Mazda, creating a nativity myth that included a miraculous conception at which all of the good creation rejoiced, while the forces of evil tried to destroy the baby (threats from which Zoroaster was saved miraculously). He was perceived as a world savior sent by God, and as it became clear that the final great battle was not imminent, the legend grew that he would come again as the future savior to finish the work he had begun.
Although the world population of Zoroastrians has sunk to only 100,000 today, the teachings of Zoroaster had an incalculable effect on the religious thought of Judaism, Christianity, and Islam, and the repercussions of his thought can also be traced in China and India. The perception of humans as essential tools of God, giving humankind an elevated place in the struggle between good and evil, placed the religion of Iran on a moral footing not central to other religions for many centuries. And the millenarian heritage of Zoroaster—that a great cosmic battle will lead to the final establishment of order on earth—is with us still today.
(Phyllis G. Jestice in “Holy People of the World”)

For further reading I recommend "A Manual of Kshnoom"