Jesus had not died yet so no idea how Abel or Cain were Christian or as I prefer disciples of Jesus.
The Bible said He was slain from the foundation of the World. The Bible says Jesus is the Word of God which became Flesh that created all things. And Jesus said "Before Abraham was, I Am."
So who was it that talked to Cain if it wasn't the Christ, the Word of God which became Flesh? And who did Cain and Abel offer sacrifices to if it were not the Christ, the Word which became Flesh? And who was it that created this example of God's Law to "Do not hate your brother without a cause" or to "Love your neighbor as yourself, if it were not the Christ?
1 John 3:
10 In this the children of God are manifest, and the children of the devil: whosoever doeth not righteousness is not of God, neither he that loveth not his brother.
11 For this is the message that ye heard from the beginning, that we should love one another.
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.
Matt. 23:
33 Ye serpents,
ye generation of vipers, how can ye escape the damnation of hell?
34 Wherefore, behold,
I send unto you prophets, and wise men, and scribes: and
some of them ye shall kill and crucify; and
some of them shall ye scourge in your synagogues, and persecute
them from city to city:
35 That upon you may come all the righteous blood shed upon the earth,
from the blood of righteous Abel unto the blood of Zacharias son of Barachias, whom ye slew between the temple and the altar.
If Abel wasn't the disciple of the Christ, then whose Word was he teaching?
Will you set your religious traditions aside and answer these questions? Or will you continue to just defend your religion.