They say this because, Adam and Eve rebelled against the very one who created them Jehovah God. Using a serpent, the spirit creature Satan the Devil deceived Eve into disobeying Jehovah, and Adam followed her. Read Genesis 3:1-6.
We cannot deny what the creator told Adam and Eve to do. He had already spelled out the consequence of disobedience, stating: “From every tree of the garden you may eat to satisfaction. But as for the tree of the knowledge of good and bad you must not eat from it, for in the day you eat from it you will positively die.” Genesis 2:16, 17
Mankind had a problem when they disobeyed and sinned against the one who created them. The original sin created a very critical problem for mankind. Adam began life as a perfect human. Therefore, his children could have enjoyed perfect everlasting life. However, Adam sinned before he fathered any children. The entire human race was still in his loins when he received the sentence: “In the sweat of your face you will eat bread until you return to the ground, for out of it you were taken. For dust you are and to dust you will return.” Genesis 3:19 So when Adam sinned and began to die as God said he would, all mankind was sentenced to death along with him. The apostle Paul later wrote: “Through one man [Adam] sin entered into the world and death through sin, and thus death spread to all men because they had all sinned.” Romans 5:12 because of the original sin, children that were supposed to be born perfect with endless life ahead of them were brought forth with sickness, old age, and death as their prospect. “That is not fair,” “We did not choose to disobey God Adam did. Why should we lose our prospect for everlasting life and happiness?” We know that if a court of law were to put a son in prison because his father had stolen a car, the son could rightly complain: “That is not just! I did not do anything wrong.” Deuteronomy 24:16. The Devil struck very early in the history of the human race before any children had been born. The moment Adam sinned, an important question was, What will Jehovah do about the children that Adam and Eve will have?
Jehovah God did what was just and fair. “Far be it from the true God to act wickedly, and the Almighty to act unjustly!” declared the righteous man Elihu. (Job 34:10) Also concerning Jehovah, the prophet Moses wrote: “The Rock, perfect is his activity, for all his ways are justice. A God of faithfulness, with whom there is no injustice; righteous and upright is he.” Deuteronomy 32:4 The solution that the true God provided to the problem created by Adam’s sin does not take away our opportunity for everlasting life on a paradise earth.
Jehovah said to Satan: “I shall put enmity between you and the woman [God’s heavenly organization] and between your seed [the world under Satan’s control] and her seed [Jesus Christ]. He will bruise you [Satan] in the head and you will bruise him in the heel [Jesus’ death].” (Genesis 3:15) This was the first prophecy of the Bible, Jehovah alluded to his purpose to have his heavenly spirit Son come to earth to live as the perfect man Jesus and then die to be bruised in the heel in a sinless state.
What about the condemnation that the entire human race came under because of that sin? A death was needed to atone for their sins. But whose death could justly cover the transgressions of all mankind? God’s Law to the ancient nation of Israel required “soul for soul [or, life for life].” (Exodus 21:23) According to this legal principle, the death covering mankind’s transgressions would have to be of a value equal to what Adam had lost. Only the death of another perfect man could pay the wages of sin. Jesus was such a man. Indeed, Jesus was “a corresponding ransom” for the saving of all redeemable mankind descended from Adam.—1 Timothy 2:6; Romans 5:16, 17. That's why people say: Jesus' death was payment for sin. He gave his life in behalf of mankind so that they could approach our heavenly father with freeness of speech. Jesus’ death is a means of rescuing “everyone exercising faith” in him from sin, disease, old age, and death itself. (John 3:16) Are you thankful to God for making this loving arrangement for our salvation?