Abel's Faith in the Coming Messiah

In the course of time Cain brought to the Lord an offering of the fruit of the ground, and Abel also brought of the firstborn of his flock and of their fat portions. And the Lord had regard for Abel and his offering, but for Cain and his offering he had no regard.
Genesis 4:3-5

By faith Abel offered to God a more acceptable sacrifice than Cain, through which he was commended as righteous, God commending him by accepting his gifts. And through his faith, though he died, he still speaks.
Hebrews 11:4

Abel's sacrifice involved blood and therefore testified to the death of a substitute. He was coming to God as God had shown he must be approached. When God killed animals in the Garden of Eden and then clothed Adam and Eve with their skins, God was showing that, because sin means death, innocent victims must die in order that sinners might be pardoned. The sacrifice pointed forward to Christ. When Abel came with the offering of blood he was believing God and was looking forward to the provision of the deliverer. When Cain brought his fruit he was rejecting that provision.

--James Montgomery Boice, as quoted by Anthony Carter in Blood Work, page 8