Our sages say that there have been “two thousand years of Torah.” RaSHI says that these began in the days of Abraham. But if Torah has existed since Abraham’s time, what did Israel receive [at Sinai]? Yes, there was Torah in Abraham’s’ day, but it was dressed in garments of the snake’s skin, as Scripture says: God made for the man and his wife garments of skin, and He dressed them ((Gen. 3:21)…The essence  of receiving Torah is that the poison [of the snake] pass out of you and the “skin” turn into light with an alef.”

Rabbi Menaḥem Naḥum of Chernobyl, Me’Or Einayim, parshat Yitro, Green translation, Light of the Eyes, pp.435-36