Before the world was created, God hid the Torah. Not in a vault, not in a distant heaven — hidden in the fabric of things, waiting for the right person to find it. And then Abraham came. The rabbis said he unlocked it.
The proof text is from Genesis: "Because Abraham obeyed My voice, and kept My charge, My commandments, My statutes, and My laws" (Genesis 26:5). The verse lists everything — charge, commandments, statutes, laws. Rabbi Yonatan said in the name of Rabbi Shimon ben Yochai that Abraham even kept the laws of cooking. How? He had no written Torah. Sinai was centuries away. The rabbis' answer: Abraham studied God through the world. He intuited the commandments the way a gifted reader intuits the author's meaning before they've reached the end of the book.
The light the rabbis were chasing here was the hidden light of the first day — the primordial light created before the sun, immediately concealed for the righteous to find at the end of days (a concept explored in Bereshit Rabbah 11:2 and other midrashim). Torah was that light in textual form. And Abraham, walking in a world before revelation, saw it anyway. This is what makes him the father of faith — not that he followed instructions, but that he found the instructions before they were given, because he was paying attention to the right things.