In a not-widely-known piece on Bereishis 14:18 ("MalkiTzedek... priest to El Elyon"), the Ohr Hachaim says the following:
ואומרו לאל עליון, להיות שהיה להם אלוהות אמצעיים כידוע לכסילים אמר כי זה כהן לאל עליון על כל האלוהות שהיו להם אז
"[The verse] says "El Elyon", since they had intermediate deities as is known for fools, it says that he was the priest to the God that was above all the gods that they had in those days..."
This would answer the question asked by many, which is how Avraham was different from the monotheism that preceded him, known in traditional sources as "Yeshivat Shem Ve'Ever". Presumably, his uniqueness was that he completely rejected all the lesser gods. It is interesting that he seems to respect MalkiTzedek. In fact, rabbinic tradition, fully upheld by the Ohr Hachaim himself, identifies MalkiTzedek as Shem, son of Noah, teacher of Avraham, Jacob and Joseph, and paradigm of righteousness.
As the disciple of MalkiTzedek/Shem, Avraham forged a new path. Judaism, according to the Ohr Hachaim, developed from a monolatrous Canaanite cult devoted to El Elyon.