For the past several centuries, Christian Theologians point to the story of Adam and Eve as the Origin of Sin. This story proceeds that after God created Adam and Eve he also gave them instruction not to eat of the Tree of the Knowledge of Good and Evil. A serpent, identified later in the Bible as Satan, persuaded Eve to eat of the fruit of this tree which God told them not to eat. She gave some fruit to Adam, and he also ate of it. They gained the Knowledge of Good and Evil, and then knew they were naked and were ashamed, and hurried to clothe themselves. God found them and, because they had disobeyed Him, cast them out of the Garden of Eden. Mercifully, He also bestowed the gift of death, so that they would not live forever, separated from Himself.
This sin is believed to have been transmitted down the generations, a sin which only the sacrifice of Jesus could rectify. The Bible says that sin entered the world through one man, Adam. Sin is nothing more than disobedience of God. It is often argued that God created evil, but that could not be, since God is perfect. He created mankind with free will, which included the ability to choose to disobey Him. Without that ability to choose, man could never have chosen to love God; rather, we would have been like little wind-up dolls that always say, "i love you", without really meaning it. With Adam's choice, sin entered the world and with it death.'
We are born with the stain of that original sin, and our nature is to satisfy ourselves rather than obey God. But God cannot tolerate sin or be in its presence. We sinners have to have another way of getting to God's abode, which we call heaven. God so loved the (people in the) world that He gave His only Son to die a sacrificial death, to atone for man's sins, and to provide the redemption that man needs in order to get to heaven and live forever in God's presence. But He placed a condition on that redemption: believe in Jesus as the Savior, the One capable of forgiving one's sins. That belief leads to repentance and a desire to please God.