Abraham, 1813-1638 BCE, is considered the first Jew.
A native of Mesopotamia, he rejected the idolatrous ways of his ancestors and contemporaries; he was the first person to use his own cognitive abilities to discover and recognize the one G‑d. He then actively publicized his newfound monotheistic beliefs among his fellow citizens.
Eventually G‑d revealed Himself to Abraham and commanded him to leave his ancestral home and travel to the land of Canaan (later known as the Land of Israel). G‑d then promised this land as the eternal inheritance of his descendants.
Abraham took upon himself the observance of the mitzvot (divine commandments) and at the age of 99 he followed G‑d's command to circumcise himself and the men of his household.
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See also "Jews" or "Israelites"?