Abel

"It was because of his faith that Abel made a better sacrifice to God than Cain, and he had evidence that God looked upon him as a righteous man, whose gifts He could accept. And though Cain killed him, yet by his faith he still speaks to us today."

What does Abel say to us today?

Faith is necessary to pleasing God.
Faith--that God is and that He is worth seeking--gives understanding.
The sacrifice of the righteous man is well-pleasing in His sight.
The faithful should expect acts of animosity from the self-righteous.
There are consequences: Adam and Eve understood "you will surely die." Cain understood that sin separates, that the eyes of the Lord are everywhere beholding the evil and the good, that ignoring God's plea for repentance and letting sin fester in one's heart is catastrophically destructive. Abel discovered that death is the doorway to life. He was the first to hear, "Well done, good and faithful servant."
You can kill the "messenger" but the message still speaks. Forever.
God rewards the faithful. Think of how long God has been using Abel's testimony to reach out to men.