Are people today still making Cain's mistake?
Yes. People today still repeat Cain's mistake. Cain did not believe what God said; he rationalized and deceived himself, assuming God would accept him without the blood. The human race has not changed one iota. Salvation still requires taking God at His Word by faith, and without the shedding of blood there is no remission.
Les explains that Cain's problem was that he did not believe what God said. God said, "Bring Me a blood sacrifice, and I'll accept you," but Cain rationalized. A man offered the definition of rationalization as self-deception, and Les agreed. Cain deceived himself, saying, "Surely God will accept me without the blood," but God did not. Abel, by contrast, did what God said. Hebrews 11 says by faith Abel offered unto God a more excellent sacrifice than Cain, his faith showing he was doing what God said to do. So Abel was accepted, and Cain was rejected. Les then says people are doing the same thing today; the human race hasn't changed one iota. Without the shedding of blood there is no remission, and without faith there is no salvation; faith is taking God at His Word.