It has been theorized by many as to the tree of the knowledge of good and evil. What was its fruit? What was original sin? I think that these are the wrong questions. It was not a particular sin as we know but a choice to rebel against the command of the Lord our God. It is a deciding for oneself what is good and what is evil. It is making oneself Lord rather than listening to the word of God. We should remember that Genesis 1-3 was written by Moses through whom the law was given.
The man and woman were to have children and multiply. They were to fill the earth and subdue it. They were to rule over the creatures of the earth. However, they choose to listen to the creature, rather than the creator. They choose to do what they wanted to do rather than have eternal life in the garden. The apostle John said in 1 John 3:4, “Sin is lawlessness.”
Lawlessness is transgression of the law. The apostle Paul wrote, Romans 5:18-21, “So then as through one transgression there resulted condemnation to all men, even so through one act of righteousness there resulted justification of life to all men. For as through the one man’s disobedience the many were made sinners, even so through the obedience of the One the many will be made righteous. The Law came in so that the transgression would increase; but where sin increased, grace abounded all the more, so that, as sin reigned in death, even so grace would reign through righteousness to eternal life through Jesus Christ our Lord.”