They do not know the vicious nature of sin. Sin can harden the heart. They do not understand the purpose of man's existence. The Lord's people must be alert to the power of evil.
Evil can be deceptive.
Evil can rob one of real life. The Lord's disciples must be alert to opportunities for service. The Lord's disciples must be alert to the fact of the Lord's coming. There are many examples of living faith set forth in Hebrews chapter 11. Obey His commandments.