Genesis 26:5 says that God did all He did for Avraham "because Avraham heeded what I said and did what I told him to do — he followed my mitzvot, my regulations and my teachings.” This answers the question of why did God choose Abraham.

Avraham knew about God and all His ways because Shem the son of Noah actually lived longer than Avraham and taught him. We have every reason to believe that Avraham’s "religion" was the Bible and that God chose him because he followed God in the midst of a pagan land. Joshua 24:2 says that Avraham’s father and ancestors served other Gods but not that Avraham did.