Genesis 4:1-2 And Adam knew Eve his wife; and she conceived, and bare Cain, and said, I have gotten a man from the Lord. And she again bare his brother Abel. And Abel was a keeper of sheep, but Cain was a tiller of the ground.
Adam and Eve had their first children. Now the wording on this seems to say that Abel was a twin because it doesn’t say that Adam knew her again like most verses. The Greek word says that she “continued” to bare Abel. Which again, sounds like a twin. However, it is my belief that they are not twins because everywhere else in the bible states who are twins. Why wouldn’t this be the same way? I suppose it is still possible they actually were twins but, it is more likely that they were born very close together. Adam and Eve, being new to sexual desires, were probably unaware of that causing pregnancy.
Genesis 4:3-8 And in process of time it came to pass, that Cain brought of the fruit of the ground an offering unto the Lord. And Abel, he also brought of the firstlings of his flock and of the fat thereof. And the Lord had respect unto Abel and to his offering: But unto Cain and to his offering he had not respect. And Cain was very wroth, and his countenance fell. And the Lord said unto Cain, Why art thou wroth? and why is thy countenance fallen? If thou doest well, shalt thou not be accepted? and if thou doest not well, sin lieth at the door. And unto thee shall be his desire, and thou shalt rule over him. And Cain talked with Abel his brother: and it came to pass, when they were in the field, that Cain rose up against Abel his brother, and slew him.
The first sacrifice was when God killed an animal for it’s skin to cover the naked Adam and Eve. From then on, a blood sacrifice was to be made to cover sin. Life for a life.
Hebrews 9:22 And almost all things are by the law purged with blood; and without shedding of blood is no remission.
It is obvious that Adam and Eve did offerings and taught their children to do the same. Cain being a tiller of the ground brought a vegetation offering while Abel, being a keeper of the sheep, brought an animal offering. Neither of these offerings mention it is the best of the crop or flock. Abel’s offering is mentioned as the firstling, the first of the newest flock. God was pleased with Abel’s offering and not with Cain’s offering. God did allow grain offerings, however, they did not atone for sin. Usually grain offering were accompanied with a burnt offering. The burnt offering first, which was an animal consumed in the fire, and then the grain offering, which only part of it went into the fire and the rest given to the priests. This was equivalent to our modern day christian tithing. Of course, I am only giving a crash lesson here because the offerings required certain actions and all that will need a whole other study.