Abraham was the first monotheist. According to the midrash, he arrived at the realisation of G-d’s oneness after a thorough intellectual inquiry. In considering the sun that rules during the day, and the moon at night, he realised neither ruled completely; hence, they must have a Creator who created them. Through this reflection, he raised himself up above the idolatrous beliefs of his countrymen. His newfound belief could only be nurtured, by removing himself from his environmental milieu, so that he would not continue to be influenced by idolatry. So that when G-d called him to lech lecha (literally, go forth to yourself), he was ready to go.
Yet, not until he departed with Sarah, Lot (his nephew), and his entourage of souls (converts) that they had acquired, did H’Shem appear to him in Shechem. The question is asked, why did H’Shem first speak (vayomer) to Abraham, when he was first called (Genesis 12:1), then appeared (vayeira) and spoke to Abraham in Shechem (Genesis 12:7)? Why is there a differentiation noted in these passages? A direct answer is not given. Yet, accompanying the vision, Abraham was granted greater insight into the nature of H’Shem. For this reason, out of gratitude towards H’Shem, Abraham built the mizbeach (altar) there (Zohar; Sheini Luchot HaBerit). I would add that there is a progressive revelation given to Abraham, beginning with his intellectual realisation. Next, he is called directly by H’Shem, then, after he takes that first step on the journey “to a land that I will show you,” H’Shem appears to him.
Abraham’s initial call, “Get thee out of thy country, and from thy kindred, and thy father’s house” (Genesis 12:1), compelled him to separate from his familiar environs. He became known as an “ivri,” mraning, “other side,” because he was from the other side of the Euphrates River. In terms of his belief, he was on one side of the moral sphere, and the world was on the other side. Incidentally, the transliteration of the word, ivri is Hebrew. Abraham was the first Hebrew, thus becoming the progenitor of the Hebrew people, whom later became known as the Israelites. Yet, the blessings given to Abraham would also include all the families of the earth being blessed through him (Genesis 12:2).