The following An excerpt from John MacArthur's The MacArthur New Testament Commentary on Romans 5.
Therefore, just as through one man sin entered into the world, (Rom. 5:12a)
Therefore connects what follows with what has just been declared, namely, that as believers we have been reconciled to God by the sacrifice of His Son Jesus Christ (vv. 8–11). Now Paul begins the analogy of Christ with Adam, the common principle being that, in each case, a far-reaching effect on countless others was generated through one man.
In the case of Adam, it was through one man that sin entered into the world. It is important to note that Paul does not say that sin originated with Adam but only that sin in the world, that is, in the human realm, began with Adam. Sin originated with Satan, who “has sinned from the beginning” (1 John 3:8). John does not specify when that beginning was, but it obviously was before the creation of Adam and Eve, because they were tempted by Satan.
After He placed Adam in the Garden of Eden, “the Lord God commanded the man, saying, ‘From any tree of the garden you may eat freely; but from the tree of the knowledge of good and evil you shall not eat, for in the day that you eat from it you shall surely die’ ” (Gen. 2:15–17). Adam was given but one, simple prohibition by God, yet the consequence for disobedience of that prohibition was severe.
After Eve was created from Adam and joined him in the garden as his wife and helper, Satan tempted her to doubt and to disobey the command of God. She, in turn, induced her husband to disobey, and they sinned together. But although Eve disobeyed first, the primary responsibility for the sin was Adam’s, first of all because it was to him that God had directly given the command, and second because he had headship over Eve and should have insisted on their mutual obedience to God rather than allow her to lead him into disobedience.
The one command was the only point of submission to God required of Adam. Except for that single restriction, Adam had been given authority to subjugate and rule the entire earth (Gen. 1:26–30). But when Adam disobeyed God, sin entered into his life and generated a constitutional change in his nature, from innocence to sinfulness, an innate sinfulness that would be transmitted to every one of his descendants.
Paul’s argument begins with the assertion that, through Adam, sin entered into the world. He does not speak of sins, plural, but of sin, singular. In this sense, sin does not represent a particular unrighteous act but rather the inherent propensity to unrighteousness. It was not the many other sinful acts that Adam eventually committed, but the indwelling sin nature that he came to possess because of his first disobedience, that he passed on to his posterity. Just as Adam bequeathed his physical nature to his posterity, he also bequeathed to them his spiritual nature, which henceforth was characterized and dominated by sin.
God made men a procreative race, and when they procreate they pass on to their children, and to their children’s children, their own nature-physical, psychological, and spiritual.
John Donne wrote these well-known lines in his Meditation XVII,
No man is an Island, entire of itself; every man is a piece of the Continent, a part of the main; if a clod be washed away by the sea, Europe is the less, as well as if a promontory were, as well as if a manor of thy friends or of thine own were; any man’s death diminishes me, because I am involved in Mankind; And therefore never send to know for whom the bell tolls; It toils for thee.
Mankind is a single entity, constituting a divinely ordered solidarity. Adam represents the entire human race that is descended from him, no matter how many subgroups there may be. Therefore when Adam sinned, all mankind sinned, and because his first sin transformed his inner nature, that now depraved nature was also transmitted to his posterity. Because he became spiritually polluted, all his descendants would be polluted in the same way. That pollution has, in fact, accumulated and intensified throughout the ages of human history. Instead of evolving, as humanists insist, man has devolved, degenerating into greater and greater sinfulness.
Ancient Jews understood well the idea of corporate identity. They never thought of themselves as isolated personalities or as a mass of separate individuals who happened to have the same bloodline as their families and fellow Jews. They looked at all other races in the same way. A given Canaanite or Edomite or Egyptian was inextricably connected to all others of his race. What one of them did affected all the others, and what the others did affected him-in a way that is difficult for modern, individual-oriented man to comprehend.
It was on that basis that God frequently punished or blessed an entire tribe, city, or nation because of what a few, or even just one, of its members did. It was in light of that principle that Abraham asked the Lord to spare Sodom if only a few righteous people could be found there (Gen. 18:22–33). It was also on the basis of that principle that God held all Israel accountable and eventually destroyed Achan’s family along with him because of that one man’s disobedience in keeping for himself some of the booty from Jericho (see Josh. 7:1–26).
The writer of Hebrews knew that his Jewish readers would understand his statement about the tithes that Levi paid to Melchizedek. “Without any dispute,” he declared, “the lesser is blessed by the greater. And in this case mortal men receive tithes, but in that case one receives them, of whom it is witnessed that he lives on. And, so to speak, through Abraham even Levi, who received tithes, paid tithes, for he was still in the loins of his father when Melchizedek met him” (Heb. 7:7–10; cf. vv. 1–3; Gen. 14:18–20). In other words, although Melchizedek lived many years before Levi, the father of the priestly tribe, was born, along with all other descendants of Abraham, Levi, by being in the seed in Abraham’s loins, shared in the tithe paid to the ancient king.
In the same way, although with enormously greater consequences, the sin of Adam was passed on to all of his descendants. When he sinned in the Garden of Eden, he sinned not only as a man but as man. When he and his wife, who were one flesh (Gen. 2:24), sinned against God, all of their descendants-that is, the entire human race in their loins-would share in that sin and the alienation from God and subjection to death that were its consequence. “In Adam all die,” Paul explained to the Corinthians (1 Cor. 15:22). As far as guilt is concerned, every human being was present in the garden with Adam and shares in the sin he committed there.
The fact that Adam and Eve not only were actual historical figures but were the original human beings from whom all others have descended is absolutely critical to Paul’s argument here and is critical to the efficacy of the gospel of Jesus Christ. If a historical Adam did not represent all mankind in sinfulness, a historical Christ could not represent all mankind in righteousness. If all men did not fall with the first Adam, all men could not be saved by Christ, the second and last Adam (see 1 Cor. 15:20–22, 45).