First of all, the Bible never says that Adam and Eve took a bite out of an apple. The fruit they ate from the tree of knowledge of good and evil is never identified in Sacred Scripture. Artists over the centuries have used the apple merely out of habit and tradition. What kind of fruit they ate is irrelevant. The point is that Adam and Eve disobeyed God.
Original sin is the term used to describe the first sin of our first parents. As prototypes of the human race, their sin had consequences—not just for them, but for their descendants. It is called “original” since it was the first sin committed by any human. It was a sin since it was a deliberate and voluntary act against the will of God.
The effects of original sin are significant. If Adam and Eve had been exposed to radioactive material, they would have passed the effects on genetically to subsequent generations. Spiritually, their sin was transmitted in the same way. We inherit their sin as much as we inherit our genes. For us, original sin is not that we committed a sin while in the womb of our mother. Rather, it means we inherit a proclivity to sin; we are born with a tendency to sin. When you were born, you did not have childhood diseases like measles or chicken pox, but you were born with a vulnerability and susceptibility to these nasty illnesses. When your parents took you to the pediatrician, who inoculated you with vaccines, those vaccines helped your body to fight off these diseases should you ever be exposed to them.
Original sin is like being born without a resistance to temptation. When God created the first man and woman, He endowed them with sanctifying grace, which makes a person holy. Though sanctifying grace does not make you sinless, it makes you spiritually strong so you can better fight temptations to commit sin and evil.
Original sin wounded human nature like a viral or bacterial infection can wound a physical body. Disobeying God is more than just breaking divine law—it is dangerous to the health of your soul. Sin is as much a spiritual disease as it is an act of defiance against the law of God.
The parents of the human race infected their spiritual health with sin, and that first sin wounded their human nature and, by inheritance, ours. First, there was the loss of sanctifying grace, which meant their descendants would be born vulnerable to the temptation to sin. Second, their souls were wounded, which resulted in a darkened intellect, a weakened will, and a disordering of the lower passions (emotions like happiness, sadness, etc.). This is why everyone at some time, regardless of intelligence, can have trouble thinking clearly when it comes to moral dilemmas presented while under great stress and strain. Then there are times when we know what we should do, but we just lack the resolve or perseverance to see it through. (Ask anyone who has been on a diet or has tried to give up smoking.) Even the Bible tells us that “the spirit is willing but the flesh is weak” (Matthew 26:41).
The next consequence of original sin is the disordering of the lower passions. God did not create robots, androids, or Vulcans; He created humans who experience emotions as part of their human nature. Jesus Christ, who was a divine person (second Person of the Holy Trinity) had a fully human nature (with emotions) and a fully divine nature. The only difference is that His human nature was not wounded since He was not conceived or born in original sin like the rest of us. His emotions were always under the control of his intellect and will. Thanks to original sin, the rest of us often have to wage a battle with our own passions and emotions to keep them in line.
Death and the loss of heaven were the final consequences of original sin. God had given mankind the preternatural gift of immortality, and the disease of sin killed that. The gravity of the offense is measured by the dignity of the person. If I slap my brother, it is wrong. If I slap my mother, it is worse because she is my mother and deserves more respect and honor. Disobeying God is a slap in His face, and since His dignity as the Almighty Lord of Heaven and Earth is infinite, the offense against Him is equally infinite. Original sin meant humanity needed a savior and a redeemer.