Original sin is a result of Greek dualism (matter is evil, our spirit is good, life is about shedding our evil bodies and going to a spiritual heaven). George Eldon Ladd's book details how Greek philosophy changed Christianity. Here is a short explanation.
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Augustine was challenged by the question that philosophers inevitably posed to Christians: “How could sin have entered the world, if God is good?” Augustine sought to answer this challenge and in so doing adopted many of the philosophers’ ideas.
The result, as evidenced by his writings, was that Augustine reinterpreted the Bible in light of philosophy. With respect to original sin, he understood the account of Adam and Eve as a description of humanity’s fall from grace. They sinned and were punished by God, and thus all subsequent humanity, being at that time biologically present within Adam, was party to the sin. The idea of innate sin and guilt became a widespread doctrine, as is shown by the following words from a popular American schoolbook used in the 17th and 18th centuries: “In Adam’s fall, we sinned all.”
But Augustine did not devise the concept of original sin. It was his use of specific New Testament scriptures to justify the doctrine that was new. The concept itself had been shaped from the late second century onward by certain church fathers, including Irenaeus, Origen and Tertullian. Irenaeus did not use the Scriptures at all for his definition; Origen reinterpreted the Genesis account of Adam and Eve in terms of a Platonic allegory and saw sin deriving solely from free will; and Tertullian’s version was borrowed from Stoic philosophy.
Though Augustine was convinced by the arguments of his earlier patristic peers, he made use of the apostle Paul’s letters, especially the one to the Romans, to develop his own ideas on original sin and guilt. Today, however, it is accepted that Augustine, who had never mastered the Greek language, misread Paul in at least one instance by using an inadequate Latin translation of the Greek original.
In Romans 5, Paul addresses the matter of sin. In verse 12 he states, “Therefore . . . sin came into the world through one man, and death came through sin, and so death spread to all because all have sinned” (NRSV). Later in the chapter, Paul juxtaposes the sin of Adam with the righteousness of Christ: “Just as by the one man’s disobedience the many were made sinners, so by the one man’s obedience the many will be made righteous” (Romans 5:19). In contrast to his contemporary theologians, Augustine drew from his reading of these scriptures that sin was passed biologically from Adam to all his descendants through the sexual act itself, thus equating sexual desire with sin. But why should he have reached this interpretation when marital sexual relations in Jewish society at the time of Christ and Paul were considered honorable and good?
Augustine’s outlook on sex was distorted by ideas from the world outside the Bible. Because so much philosophy was based on dualism, in which the physical was categorized as evil but the spiritual as good, some philosophers idealized the celibate state. Sexual relations were physical and therefore evil.
Augustine’s association with Neoplatonic philosophers led him to introduce their outlook within the church. This had its effect in the development of doctrine. For example, Jesus was considered immaculately conceived—without sin in that His Father was God. But because His mother, Mary, had a human father, she suffered the effect of original sin. In order to present Jesus Christ as a perfect offspring without any inherited sin from either parent, the church had to find a way to label Mary as sinless. They did this by devising the doctrine of her immaculate conception, though this inevitably leads to further questions.
Other babies were not so fortunate. Some eight centuries later the Catholic theologian Anselm extended the implications of Augustine’s concept of original sin and claimed that babies who died, did so as sinners; as sinners, they had no access to eternal life but were condemned to eternal damnation.
The world from which Paul came had a very different view of sexual relations, especially within marriage. Sex was not evil; it was part of the physical creation that God had decreed was good (Genesis 1:31). The writer of the Epistle to the Hebrews supports this view in describing the marital bed as “undefiled”—that is, pure or sacred; in other words, the sexual act did not impair a person’s relationship with God (Hebrews 13:4). The apostle Paul takes the idea further in his First Epistle to the Corinthians, where he instructs married couples not to defraud one another but to render appropriate conjugal dues. He states that in the sexual relationship each partner should focus on providing benefit to the other, not just on his or her own satisfaction. Hence Augustine’s view of sex as sin does not match New Testament teaching. Nor does it coincide with the Old Testament statement that a child does not carry its father’s sin (Ezekiel 18:19–20).
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The blood of the high priest has special value. In agreement with this principle, Zech. 3 uses all the symbolism of a defiled human high priest Joshua and then speaks mysteriously of the Branch in connection with which “I will remove the sin of this land in a single day” (Zech. 3:9).
The Shadow of Christ in the Law of Moses | The Works of John Frame and Vern Poythress
Mankind was separated from God after the fall. Death is separation from God, so Adam did "die". Whatever he did was futile, with no lasting, "eternal" value.
Joshua brought Israel into the Promised Land, but he did not give them rest. The Land is a type of mankind and the promised Land is the first Adam. Christ's blood cleansed mankind, and those who abide in Christ find rest in the second Adam.
The first type was not rest, but it was the interim solution for Israel. Staying in the Land required work, resulting in the blessings of Deuteronomy 28. Disobedience led to judgment, wrath and exile. Even surrogate populations were obliged to meet the requirements:
2 Kings 17:19Also Judah did not keep the commandments of the LORD their God, but walked in the customs which Israel had introduced. 20The LORD rejected all the descendants of Israel and afflicted them and gave them into the hand of plunderers, until He had cast them out of His sight.
[bless and do not curse][bless and do not curse][bless and do not curse][bless and do not curse][bless and do not curse][bless and do not curse]21When He had torn Israel from the house of David, they made Jeroboam the son of Nebat king. Then Jeroboam drove Israel away from following the LORD and made them commit a great sin. 22The sons of Israel walked in all the sins of Jeroboam which he did; they did not depart from them 23until the LORD removed Israel from His sight, as He spoke through all His servants the prophets. So Israel was carried away into exile from their own land to Assyria .
[bless and do not curse][bless and do not curse][bless and do not curse][bless and do not curse][bless and do not curse][bless and do not curse]24The king of Assyria brought men from Babylon and from Cuthah and from Avva and from Hamath and Sepharvaim, and settled them in the cities of Samaria in place of the sons of Israel. So they possessed Samaria and lived in its cities. 25At the beginning of their living there, they did not fear the LORD; therefore the LORD sent lions among them which killed some of them. 26So they spoke to the king of Assyria, saying, “The nations whom you have carried away into exile in the cities of Samaria do not know the custom of the god of the land; so he has sent lions among them, and behold, they kill them because they do not know the custom of the god of the land.”
[bless and do not curse][bless and do not curse][bless and do not curse][bless and do not curse][bless and do not curse][bless and do not curse]27Then the king of Assyria commanded, saying, “Take there one of the priests whom you carried away into exile and let him go and live there; and let him teach them the custom of the god of the land.” 28So one of the priests whom they had carried away into exile from Samaria came and lived at Bethel, and taught them how they should fear the LORD.
Believers today live in Christ, the real Promised Land, the second Adam, redeemed mankind, walking with the Father just as Adam walked with God in the Garden.