"AND HE CALLED OUT IN THE NAME OF HASHEM" (Bereishis 12:8)
Abraham was chosen because this was his practice, and his seed was chosen in order that they continue his practice. "This nation I created for Me, that they relate My praise" (Isiah 43:21).
The praise includes the study of the Creator's kindliness which is evident from the phenomena of Creation, and also the study of His wisdom of plan-and-purpose and His power. Abraham fulfilled this function most successfully, and his entire life was devoted to this career of growing greater in Awareness of Hashem and the wisdom and kindliness of His works; and he spoke of these matters to others
1) In order to express his thoughts and thereby gain even more Awareness for himself,
2) And in order to impart this knowledge to others.
This practice which had begun in the days of Enosh (4:24) -- The beginning