One of the most prevalent errors today is a mindless optimism about human salvation.
Everybody assumes that either hell doesn't exist, or that it doesn't really matter because "everyone is going to heaven anyway".
Most “believers” run around with watered down, sugary, “feel good” versions of Christianity in their heads, a trivialized Christianity stripped of all its uncomfortable teachings.
They think that God is good and merciful, salvation is easy, and so there’s really nothing to fear about hell.