The tares sown by Satan are spread throughput the world, Church included. We don't know for sure who the reprobates are. Many non-Christians are still written in the Book of Life - I dont think there is much time left though!
"The one who sows the good seed is the Son of Man; the field is the world, and the good seed are the children of the kingdom; the weeds are the children of the evil one, and the enemy who sowed them is the devil; the harvest is the end of the age, and the reapers are angels. As the weeds are pulled up and burned in the fire, so it will be at the end of the age. The Son of Man will send out his angels, and they will weed out of his kingdom everything that causes sin and all who do evil.Matt. 13:38-41[/QUOTe ]
The reason that one can not understand that Cain was the son of the Serpent is because they can't fandom Eve having sex with the devil, the serpent.
The scripture says that Eve told God that she was seduced.
Gen 3:13 And the LORD God said unto the woman, What is this that thou hast done? And the woman said, The serpent beguiled me, and I did eat.
beguiiled
naw-shaw'
A primitive root; to lead astray, that is, (mentally) to delude, or (morally) to seduce:
1Jn 3:12 Not as Cain, who was of that wicked one, and slew his brother. And wherefore slew he him? Because his own works were evil, and his brother's righteous.
Cain was the son of that wicked one, the serpent.