Baal HaSulam, “The Arvut” (Mutual Guarantee): “The Patriarchs observed the whole Torah even before it was given.” This means that because of the exaltedness of their souls, they had the ability to attain all the ways of the Creator.
The patriarchs belonged to the pure vessels, GE (Galgalta ve Eynaim), and therefore did not need the Torah, because the Torah is the Light that corrects the Aviut (thickness) of the desire, and they were not thick but pure. The whole Torah is love and bestowal and they could observe it out of the exaltedness and the purity of their souls.
All the lower vessels, however, could not observe it just like that but needed to be equipped with sufficiently thick desires, which correct themselves by the Light that Reforms and could eventually be detached from it and receive the Torah.
Vessels of Hesed (mercy), Gevura (valor ) and Tifferet (splendor) correct themselves without the Light that Reforms, but are simply adhered to the simple Upper Light out of their purity. They knew all the Torah intuitively, since their vessels let them know about actions of bestowal. But their sons Netzach (eternal) , Hod (glory), and Yesod (foundation), all the other souls that come after them, already belong to the vessels of receiving and are therefore unable to understand, to feel, or to correct themselves by their own understanding and purity.
When the vessels are thicker, they need the Light that Reforms. First they have to reveal the full depth of the thickness of the desire to realize how bad it is on the levels of Egypt and during 400 years.
This is what Abraham asked: “I know that I belong to HGT, to vessels that are already corrected, that we are pure and that we are closer to You, but what about my sons? How will I know that they will inherit the land (of Israel)?”
The Creator answers him: “Don’t worry, they will be in exile and will accumulate all the necessary thickness, and then they will realize that they need to be connected to Me.” Abraham heard that and calmed down. The vessels of bestowal have a natural inclination to the Creator, while the vessels of receiving can only establish some attitude towards the Creator by going through exile that reveals the evil in them.
The method of correction is totally different in vessels of bestowal, in GE, and in vessels of receiving. Fathers and sons are two different levels. The sons are already egoistic as we can see.
From the 5th part of the Daily Kabbalah Lesson 6/5/14, Writings of Baal HaSulam