The main Scripture to understand on this topic is Romans 5:12: “Therefore, just as through one man sin entered the world, and death through sin, and thus death spread to all men, because all sinned” 

Acts 17:26 “And He has made from one blood every nation of men to dwell on all the face of the earth” because we all come from the same original man we inherited his flawed makeup of nature. Sin entered the world through the first man and spread to all his offspring- this happened by Adam sinning. Death is a result of sin, it was the result of sin. It occurred two ways. First God told Adam the day you eat of it you will die, Adam was separated from the Lord he died spiritually and was severed in his relationship. Second- He died physically, which was a result of his nature change. Aging and death began on that day. Both were a result from his disobedience in eating the fruit of the tree. 1 Cor.15:56: “the sting of death is sin,..”

Romans 5:18-19 says, "through one man's trespass, judgment came to all men, for by one man's disobedience all were made sinners."  We are all born with a built-in death warrant, which is our sin nature, because we have inherited this sin nature from Adam we are already born spiritually dead (separated from God) and die physically. 1 Cor. 15:22: “For as in Adam all die, even so in Christ all shall be made alive.” Death came by Adam and the resurrection can only come to mankind by believing in Jesus Christ.

Later, God brought the law for us to see our sin measured by his standard. Gal 3:19 “Why the Law then? It was added because of transgressions,.., until the seed should come to whom the promise had been made.” Rom. 5:20-21 “Moreover the law entered that the offense might abound. But where sin abounded, grace abounded much more, so that as sin reigned in death, even so grace might reign through righteousness to eternal life through Jesus Christ our Lord.”

Because of our fallen humanity we cannot act apart from our nature. We can see the sin nature at work even in a child who is not trained in behavior as a child disobeys and is selfish naturally, they do not need to learn this. The Bible says we act on our nature.  The Psalmist says Ps.58:3 “the wicked go astray from the womb, they err from their birth speaking lies. Job 15:14"What is man, that he could be pure? And he who is born of a woman, that he could be righteous? Job 25:4How then can man be righteous before God? Or how can he be pure who is born of a woman?

We don't just make up our mind to sin or make a mistake and go from being perfect to being sinful. Eph. 2:3 “and were by nature the children of wrath”

James 1:14-15 “But each one is tempted when he is drawn away by his own desires and enticed. Then, when desire has conceived, it gives birth to sin; and sin, when it is full-grown, brings forth death.”

 Sin comes from within man, it can be a reaction from what is outside a person or just what a person is thinking. John tells us we have an enemy on three fronts the world (which is fallen and under sin and the devils control) the flesh (which carries the sin nature) the devil (a personal angel that is sinful and influences mankind to love sin). Sin has its roots in the heart that influences the intellect and will and ultimately finds its expression through the body when we follow though with the desire (Prov.4:3; Mt.15:19-20; Lk.6:45; Heb.3:12 ; Jm1:14-15. The sin nature is the basis for sinful habits, it is not a single act but a process that begins in our heart. You can say the real source of mans problem is heart disease, our fallen human nature.

Jer. 17:9: "the heart is deceitfully wicked, ( incurably sick). Jesus taught that sin begins within us.   Matt 15:18-19 Jesus said, "But those things which proceed out of the mouth come from the heart, and they defile a man. "For out of the heart proceed evil thoughts, murders, adulteries, fornications, thefts, false witness, blasphemies.”

The sin nature affects the immaterial part of man, which consequently affects our outward actions. We sin because we have the nature of sin already in us.Mark is even more specific. (7:20-23) And He said, "What comes out of a man, that defiles a man. "For from within, out of the heart of men, proceed evil thoughts, adulteries, fornications, murders, "thefts, covetousness, wickedness, deceit, lewdness, an evil eye, blasphemy, pride, foolishness. "All these evil things come from within and defile a man."

The heart was representative of the innermost being of man. Jesus was affirming what Jeremiah wrote in 17:9-10: "The heart is deceitful above all things, and desperately wicked; who can know it? I, the LORD, search the heart, I test the mind, even to give every man according to his ways, according to the fruit of his doings.”

David explained that his conception had sin involved. Ps 51:5: “Behold, I was brought forth in iniquity, and in sin my mother conceived me.” We as David have a sin nature when we are born, it is not something we learn afterwards. Our actions only express what our nature is. The sin nature is the basis for all sinful habits. Without understanding the source of sin one cannot understand nor give the solution correctly. The Law is not the answer, it points to the problem, it is diagnostic but it is not the cure.

The Bible explains that we are all slaves to sin except for one- Jesus, who did not inherit the sin nature because he was conceived without the aid of a man, formed by God in the womb of a virgin. This why there was a need to bypass the sin nature of man when He took on human flesh.

That is why it makes the point of Jesus being different from conception onward: 1 Peter 2:22: "Who committed no sin, nor was deceit found in His mouth”

I Jn 3:5 And you know that He was manifested to take away our sins, and in Him there is no sin.”

There is none that do not sin John introduced Jesus as the ONLY one who can bring the solution, "Behold! The Lamb of God who takes away the sin of the world! John 1:29. Only Christ Jesus has the solution no other religion can deal with the fallen nature of man.

 There are those who claim we are not sinful and think they have been born without sin, they do not understand what sin is and do not know how holy God is.

I Jn. 1:8: “If we say that we have no sin, we deceive ourselves, and the truth is not in us”

All have sin and all do sin- that is the Bibles position from the beginning.

Prov. 14:8-9“The wisdom of the prudent is to understand his way, but the folly of fools is deceit. Fools mock at sin, but among the upright there is favor.”

The Bible never teaches we become sinners but are sinners, those who sin.

Eccl. 7:20: “For there is not a just man on earth who does good and does not sin.”

Rom. 3:9-12 “both Jews and Greeks that they are all under sin. As it is written: "There is none righteous, no, not one; There is none who understands; there is none who seeks after God. They have all turned aside; they have together become unprofitable; there is none who does good, no, not one" (for only God is called good).

This is why John writes 3:19 "And this is the condemnation, that the light has come into the world, and men loved darkness rather than light, because their deeds were evil.”

What is man’s solution to this sin nature- try to be good and do good; have your good deeds outweigh your bad deeds- in other words humanism.

What is God’s solution to this sin nature? The Bible tells us that God gave us the way back into a relationship with him, to be born again- spiritually (Jn.3:3-7).

He gave us His son and the gospel to believe in to be restored.

Rom 10:10: “For with the heart one believes unto righteousness, and with the mouth confession is made unto salvation.” One must believe not only in their mind but their spirit their innermost being to be saved.

Being spiritually reborn the sin nature is not removed (until we are glorified) but we are given a new nature, a renewed mind that rule over it.

Rom 7:21-25 “I find then a law, that evil is present with me, the one who wills to do good. For I delight in the law of God according to the inward man. But I see another law in my members, warring against the law of my mind, and bringing me into captivity to the law of sin which is in my members. O wretched man that I am! Who will deliver me from this body of death? I thank God-- through Jesus Christ our Lord! So then, with the mind I myself serve the law of God, but with the flesh the law of sin.”

As believers we train ourselves to be more righteous and changed into the image of God’s son.

Col 3:9-10 “since you have put off the old man with his deeds, and have put on the new man who is renewed in knowledge according to the image of Him who created him”

Rom 6:17 “But God be thanked that though you were slaves of sin, yet you obeyed from the heart that form of doctrine to which you were delivered.”

Eph 6:6 “not with eyeservice, as men-pleasers, but as bondservants of Christ, doing the will of God from the heart”

We now are willing to do what is right what s God’s will instead of being inclined to only do sin. This is why those who become Christian have a drastic change of life.