THE QUESTION:
The first sacrifices spoken of in Scripture are those of Abel and Cain. One was accepted the other was not. But why was that? Hebrews 11:4 says that Abel offered a "better" sacrifice than Cain did. So, what made it "better"?

LET'S TAKE A CLOSER LOOK:
Our evidence is found in Hebrews 4:4 and Genesis 4:3-6.

Hebrews 11:4 does provide us with an explanation that "by faith Abel offered to God a better sacrifice than Cain". That tells us there was a difference in the attitude of the two, a difference in their hearts.

But is there more to it than this? Is the only difference in the attitude? Was it his faith alone that caused God to "speak well of his offerings"?

Genesis 4 provides us more information.

When Cain is upset about God's not looking with favor on his sacrifice, God says to him

"Why is your face downcast?
If you do what is right, will you not be accepted?"

In other words, Cain DID do something that was "not right". But what ?

Not acting "in faith" is a legitimate answer, but it is not the whole of it.

The key is in the description of the sacrifices (v.3-4):

"Cain brought some of the fruits of the soil
as an offering to the LORD.
But Abel brought fat portions from
some of the firstborn of his flock."

What is the difference between the two? Abel took care to bring the very best --that's how the 'fat portions' and 'firstborn' are depicted later in the OT (Exodus 13:12, Leviticus 3:16).

And what of Cain? As a farmer, he could have brought the "firstfruits" of his crops (Ex 34:26)--again, the first and best (and symbolic of the whole), but that is not what he did.

Thus the form of the sacrifice maters! The form is, of course, an expression of the heart of faith. But, as both Hebrews and Genesis 4 show us, this faith is seen in what one does "by faith".

So, could Abel have offered a defective sacrifice (or even 'no sacrifice'?) "by faith"? That is, Scripturally, unthinkable --a contradiction in terms! (cf. Mal 1:7-13) It is precisely the fact that he offered the best that expresses his heart of faith.

What about the Blood?

There is a popular view of this passage that says that Abel pleased God because he offered a blood-sacrifice for atonement, whereas Cain did not. This is theologically very attractive. Scripture certainly emphasizes the need of atoning blood sacrifice for sinners.

But in fact the context says nothing about this. . . it doesn't even mention the blood, but rather the 'fat portions' and the 'first-'. Also, the term used for "sacrifice" in these verses is not a term connected with atoning blood-sacrifice, but a more general term associated with tribute and thanks. In the laws of Leviticus it is the term used for the grain offering, an offering that did not include blood (Lev 2:1ff)! So, as vitally important as atonement is, it's not the point in this particular passage. The issue is rather, giving God the best, for that is what is 'due his name'!