"On Preaching" from the Confessions (#3)

"Whenever good works are praised and the law preached, therefore, we must hold fast to these rules: that the law is not kept without Christ — as he himself has said, 'Apart from me you can do nothing' (John 15:5) — and that 'without faith it is impossible to please God' (Heb. 11:6). The teaching of the law is certainly not intended to abolish the Gospel of Christ, the propitiator. Cursed be our opponents, those Pharisees, who interpret the law in such a way that they attribute Christ’s glory to works and make of them a propitiation that merits the forgiveness of sins. It follows, therefore, that works are praised for pleasing God on account of faith, since they do not please him without Christ, the propitiator. 'Through him we have obtained access' to the Father (Rom. 5:2), not by works without Christ, the mediator."

(Tappert, The Book of Concord. Fortress Press: Philadelphia, 1959, p.147; Apology IV:269.)

This section from the Apology is a great explanation for why Abel's sacrifice was acceptable and Cain's was not (cf. Heb 11:4): Abel offered his sacrifice in Christ, while Cain's sacrifice was offered in something else.

I used this recently as a sermon illustration:

Some speculate that Eve believed that Cain was the Promised Seed (Gen 3:15); some translate her words in Genesis 4:1 "I have gotten a man - the LORD!" Perhaps Cain, too, thought that he was the Christ and looked at himself as the hope of the world and the Savior of mankind. Coming to the place of sacrifice, Cain made an offering out of the fruit of his labor, vegetation cultivated under the sweat of his brow; as the "Promised Seed", Cain made a meritorious sacrifice to God of his own good works. But as time would soon tell, Cain was not that Seed, and his sacrifice was powerless to save.

By contrast, little brother Abel was definitely not the Promised Seed. He was the second son, born without expectation, whose only hope for salvation was in the One whom God had promised. Abel's sacrifice reeked of sin; Abel slaughtered his firstborn sheep and brought the fat of his flock, reminding the Lord God of the animal sacrifice that was necessary to clothe man's nakedness after the Fall (Gen 3:21). Abel's sacrifice pointed both to man's nakedness and sin as well as to the One who covers mankind's nakedness and sin, the Promised Seed who would strive with the devil and all his evil cohorts to win forgiveness, life, and salvation for mankind. Abel's sacrifice was pleasing to God because it was offered with trust in the One who is powerful to save; Abel's sacrifice pointed to the Promised Seed, the Christ. Abel's sacrifice pointed to the One who would be lifted up as a sacrifice upon the altar of the cross for Abel's salvation and for the salvation of all mankind.