The concept of original sin? I'm not sure if I understand your question. But I'll try anyway. First of all do you understand what sin is? To understand this word, let us consider the meaning it carried in the original languages of the Bible. The Hebrew and Greek verbs commonly translated "to sin" mean "to miss" in the sense of missing a mark or not reaching a goal. What mark do all of us miss? The mark of perfect obedience to God. Now we can get into your question of why I believe in the 'concept.' I believe because it happened. How, though, was sin introduced into the world? God commanded Adam: "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) As the Creator, Jehovah God had the right to set moral standards and to define what was good and what was bad for his creatures. His command was reasonable because Adam and Eve were free to eat fruit from all the other trees in the garden. They could show their appreciation for Jehovah's rightful rulership by obeying this law instead of proudly setting their own moral standards. As many know the story progresses to Adam and Eve sinning (missing the mark of complete obediance) by eating the fruit of the Tree of Knowledge of Good and Bad. Now, Adam and Eve had to face the consequences of their actions. They did not become like God, with special knowledge. Instead, they felt ashamed and hid themselves. Jehovah called Adam to account and pronounced this 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) "In the day" that our first parents ate from the tree of the knowledge of good and bad, they were sentenced by God and died from his standpoint. Then they were expelled from Paradise and began their descent into physical death. When sin started to work in the first human couple, they could no longer pass on perfection to their offspring. Like an inscription carved in stone, sin was deeply engraved into the genes of our first parents. Thus, they could produce only imperfect offspring. Since all their children were conceived after Adam and Eve sinned, their offspring inherited sin and death.—Psalm 51:5; Job 14:4.

In the Old Testament, in Psalm 51:5; "Behold, I was shapen in inquity; and in sin did my mother conceive me." I hope this helps. Thank you and God bless you!

Far from promoting the concept of original sin the Hebrew Bible dismisses it outright. The following four teachings are from Ezekiel 18:20-23: 1.1- "THE SOUL THAT SINNETH, IT SHALL DIE” - Adam is the soul that sinned and Adam was the one to bear the loss 1.2- “THE SON SHALL NOT BEAR THE INIQUITY OF THE FATHER” - Let alone us so many generations after Adam, even Adam's immediate children do not bear Adam's sin. 1.3- “BUT IF THE WICKED WILL TURN FROM ALL HIS SINS …. HE SHALL SURELY LIVE, HE SHALL NOT DIE. ALL HIS TRANSGRESSIONS THAT HE HATH COMMITTED, THEY SHALL NOT BE MENTIONED UNTO HIM” - if a soul that has sinned repents it shall be forgiven as if sinless no matter the amount of sins committed, Adam committed just the one 1.4- “HAVE I ANY PLEASURE AT ALL THAT THE WICKED SHOULD DIE? saith the Lord God: AND NOT THAT HE SHOULD RETURN FROM HIS WAYS, AND LIVE?" - God is eager to forgive not eager to exact punishment 2- Even in the NT we see that the concept goes against the teachings of Jesus who regarded little children as sinless as he confirms in Matthew 19:14 "Suffer little children, and forbid them not, to come unto me: for of such is the kingdom of heaven" 3- The HQ confirms that Adam and Eve sinned, but says that they paid a heavy price, they got banished from the Garden. The HQ says they realised their error, repented and were forgiven by the Oft-Forgiving God.