Isaac is remembered mainly as the long awaited promised son whom Abraham was willing to sacrifice on the altar. Most images show him as a young boy, however the bible puts him at about 25 to 35. Isaac was 110 when Shem died. I believe that Isaac would have had many opportunities to meet with Shem over his life both with his father Abraham and during the 35 years between Abraham’s death and Shem’s death.
So not only would Isaac have been instructed by Abraham about how to worship and come to know God but he would have been taught by a man who passed through the flood, and knew Methuselah and Lamech who both knew Adam and Enoch. Shem also spent his life with Noah and if he is Melchizedek, priest of the Most High God I cannot imagine he had not been in the presence of God many times.
Even if he had all these witnesses Isaac would still have had to follow the pattern these men did. Just hearing about God is not knowing him. Moses tells us about two experiences where God appeared to Isaac and made the same covenant with him that he made with his father Abraham.
While the writings about Isaac are short the fruits are bountiful. The fruits tell the type of life a man has lived and the fruits are the relationship with God not the power and wealth of this world.