Targum Yonasan gives us a clue concerning the conversation that took place between the world’s first two brothers. Kayin said, “There is no Judge, and there is no Justice; there is only one world; there is no reward for the righteous and no punishment for the wicked.” Hevel, of course, disputed each point. These words led to physical violence, during which Hevel was killed. We wonder at Kayin’s hypocrisy. Here is a man who had just offered a sacrifice to Hashem, and he was distraught that Hashem was more pleased with Hevel’s sacrifice than his. If Kayin felt there was no Judge, i.e., no G-d, why did he offer a sacrifice? Apparently, he did not really believe it.
Horav Moshe Shternbuch, Shlita, suggests that Kayin was actually aware of Hashem’s existence. He only had to take one look at his father, Adam HaRishon, to see Hashem’s handiwork. His problem was that he was hurt; he was distressed that Hashem did not accept his sacrifice with favor. This is why he made his sacrilegious statements. He knew he was wrong but he was angry that his sacrifice was not accepted. Instead of introspecting to discover a reason that he was not heard, he denied Hashem’s existence. To erase the envy in his heart over his brother’s acceptance, he renounced the validity of his acceptance.
Is it any different today? All of those who deny Hashem are doing so to placate themselves. They know the truth; they just cannot handle it. It is easier to repudiate Hashem’s Torah than to admit one’s personal inadequacy.