Cain and Abel were twins although, it was not explicitly mentioned in the bible, it was implied in the verse below;
“Adam made love to his wife Eve, and she became pregnant and gave birth to Cain. She said, “With the help of the Lord I have brought forth a man.” Later she gave birth to his brother Abel. Now Abel kept flocks, and Cain worked the soil.”Genesis 4:1-2 NIV
“Later” is a term commonly used to indicate a short gap in between now and afterward. It’s unlikely that later happens in a minimum of 9 months since the process of conception to birth will take 9 months at most.
Having said the above, the case of the birth of Cain and Abel can be considered as the same case as Esau and Jacob.
This is one example of “Our thought is not God’s thought”. Majority of the society would consider the first who come out to be the eldest in fraternal twins but, God thought is different as supported by the verse below;
“Then you shall say to Pharaoh, ‘Thus says the LORD: “Israel is My son, My firstborn.” Exodus 4:22 NKJV
Jacob came out last at birth, (Gen 25:24-26). Yet, as the above verse said, Jacob who is Israel was God’s firstborn. This is the reason why, in-spite of Jacob unjustified deed to take Esau birthright, it was still honored by God because it was his in the first place.
“Once when Jacob was cooking some stew, Esau came in from the open country, famished. He said to Jacob, “Quick, let me have some of that red stew! I’m famished!” (That is why he was also called Edom. ) Jacob replied, “First sell me your birthright.” “Look, I am about to die,” Esau said. “What good is the birthright to me?” But Jacob said, “Swear to me first.” So he swore an oath to him, selling his birthright to Jacob. Then Jacob gave Esau some bread and some lentil stew. He ate and drank, and then got up and left. So Esau despised his birthright.”Genesis 25:29-34 NIV
The word used was famished which means extremely hungry, no matter how you look at it, Jacob took advantage of the life and death situation of his brother to get what he wants. Yet, in- in spite of Jacob action, the transaction of how Jacob get the birthright was still honored by God. Now, why is that? Why would God honor a fraudulent transaction?
Answer: Because the birthright was his in the first place but, since the ways of men are different from the ways of God, Jacob has to fight for it and even when it was obtained in an unjustified manner still, it was honored by God because, in fraternal twins, the one who comes out last is God’s eldest as he was the one that was formed first in the womb giving way to the next who will be formed. In short, in God’s way, the eldest is based on who was formed first in the womb and not on who came out first from the birth canal but, the one who was formed first will be the last one to come out from the birth canal. The last will be the first (Matt 20:16).
In hindsight to the case of Jacob and Esau, we can parallel it to what happened to Cain and Abel.
Gen 4:1-2, Cain came out first then next Abel.
If interpreted correctly, this was one of the reasons Abel was more favored than Cain. Think about it, why would God not accept Cain’s offering when God did not inform them yet what offering He prefer in the first place?
In analogy, suppose I’m the father of twins, they bought me a pair shoes as a gift without them being aware of what shoes I liked, one bought a pair of Nike and the other bought a pair of Adidas, I reject the pair of Adidas and accept the pair of Nike because, apparently I don’t like Adidas but before that, they were not aware of what shoes I liked or don’t liked. What could be the logical reason why I would not accept the Adidas shoes? How would the other twin know that I like Nike more than the Adidas? It’s like because the other one had guessed wrong, it was rejected. No matter how you look at it, there is no logical reason why would I not accept the Adidas, unless personally, I favored the one who gives the Nike shoes hence, I did not accept the other one.
Firstborns are favored by God and Abel was the firstborn or the first one to be fertilized in the womb but the second to come out from the birth canal because there would be no logical reason on why Cain’s offering was rejected since he did not know what God preferred. How would Cain know that God liked animals offering more than fruits when prior to the offering there was no regulation in place on what or which is more preferred offering by God? Because of this, Cain must have felt jealousy towards Abel thus, so angry that he kills him. Read Gen 4:3-8
But, if based on serpent seed interpretation where Eve and Serpent had intercourse prior to Adam to which, Eve produces a superfecundation twin, since Abel was the last one to come out, it follows that he was the one formed first in the womb due to sexual activity of Eve and Serpent, making Abel the serpent seed. Moreover, Gen 3:15 confirmed it, He will strike serpent head, implying that serpent seed will die, Abel died.
This is the reason why I say that the serpent seed doctrine is bogus teaching because, physiologically, Abel will be that product if it happens that Eve and Serpent had sex prior to Adam contrary to what they say that Cain is the product of Serpent and Eve.
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