Adam -- Adam is both the name of the original man, Adam, and a Hebrew word that means “human.” - NLTSB
Nevertheless death reigned from Adam to Moses -- Evidence that there was some kind of law that mankind was under even before the law of Moses. {see note in Romans 5:13.)
even over those who had not sinned according to the likeness of the transgress of Adam -- Adam has explicit revelation from God and Adam transgressed it. Other mankind didn’t have the same kind of special revelation from God, but had their own instinctive (Romans 2:14 NASB) moral revelation, and they transgress that. (Romans 1:18-32 and Romans 2:14-15)
similitude of Adam’s transgression -- Adam’s transgress was against special revelation, a direct command. Others violated, transgressed, their own conscience of moral law and did evil. Genesis 6:5
Adam’s transgression -- Adam’s sin was a violation of a direct positive law. Other’s sins were a violation of moral law. Thus a law existed before Sinai; because men died (spiritually). [Adam’ death did bring physical and spiritual death to mankind, and both are rectified in Jesus Christ. Paul later also makes this plain, but here he dwells on the spiritual side of death.]
The "likeness of Adam’s transgression" upon us today, is that like Adam we today transgress God’s special revelation given to us in the N.T.
death reigned ... even over those who had not sinned -- Is this Paul’s way of affirming that death came, even to "innocent" children, infants, etc. who had not sinned like Adam did. All of Adam’s descendants died, whether they had "sinned" or not, like Adam did.
those who had not sinned -- Those holding to "original sin" would argue that since infants die, this is proof they are "sinners." Alford Barnes’ response is:
(1) The apostle makes no mention of infants. He does not in the remotest form allude to them by name, or give any intimation that he had reference to them.
(a) That this was an inference which the apostle does not draw, and for which he is not responsible. It is not affirmed by him.
who is a type [pattern; figure] of Him who was to come -- Paul is going to make a contrast between what Adam did and what Christ did. Adam introduced "spiritual death" (or separation); (and physical death as well) while Christ brought reconciliation between mankind and God. [See note below on Romans 5:16-17 to see that Christ’s action accomplished "much more".]
The word “pattern” is τύπος (typos) or “type.” Moo (I:346) explains that this word originally meant “the impression made by striking something,” and that it came to mean “a form, pattern, or example.”
In the NT it refers to those OT persons, institutions, or events that are seen to have a divinely intended function of prefiguring the the age inaugurated by Christ.
who is the figure -- - τύπος tupos. “Type.” This word occurs sixteen times in the New Testament, John 20:25 (twice); Acts 7:43-44; Acts 23:25; Romans 5:14; Romans 6:17; 1 Corinthians 10:6, 1 Corinthians 10:11; Philippians 3:17; 1 Thessalonians 1:7; 2 Thessalonians 3:9; 1 Timothy 4:12; Titus 2:7; Hebrews 8:5; 1 Peter 5:3. It properly means,
(1) Any impression, note, or mark, which is made by percussion, or in any way, John 20:25, “the print (type) of the nails.”
(2) An effigy or image which is made or formed by any rule; a model, pattern. Acts 7:43, “ye took up the tabernacle of Moloch and the star of your god Remphan, figures (types) which ye had made.” Acts 7:44, “that he should make it (the tabernacle) according to the fashion (type) which he had seen,” Hebrews 8:5.
(3) A brief argument, or summary, Acts 23:25.
(4) A rule of doctrine, or a law or form of doctrine, Romans 6:17.
(5) An example or model to be imitated; an example of what we ought to be, Philippians 3:17; 1 Thessalonians 1:7; 2 Thessalonians 3:9; 1 Timothy 4:12; Titus 2:7; 1 Peter 5:3; or an example which is to be avoided, an example to warn us, 1 Corinthians 10:6, 1 Corinthians 10:11.
who was to come -- The expression “he who was to come” is often used to denote the Messiah. As applied to him, it means that there was in some respects a similarity between the results of the conduct of Adam and the effects of the work of Christ. - BN
Note: The dash "— " at the end of this verse in the NIV is to show Paul’s break in thought (vs. 13-14): "He turns aside from his main argument to deal with a related matter." - NIVZSB.
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Alternate Interpretation - "death" = physical death; [Jack Cottrell, CPNIV]
Nevertheless death reigned -- Paul seems to making a play on this word "death" and his audience now sees he turns to the meaning of physical death.
death reigned from the time of Adam to the time of Moses . . This is clearly a reference to physical death, which is personified as a tyrant having everyone under its power in the period in question (and in all other times as well; see Romans 5:17). To depict death as a reigning monarch emphasizes its universal scope, its oppressive domination, and its inescapable certainty. - CPNIV
even over those who did not sin by breaking a command, as did Adam -- This is the key point of the whole argument in vv. 13-14; this is what proves the fact that Adam’s sin brought death upon us all: death reigned, even over this group.
even over them [those] -- Over all the generations from Adam to Moses.
Evidence all those generation had a "law" even before the "Law of Moses."
even -- The word “even” (kai) is important, because it focuses the argument on a group that is more narrow than the general population of the earth (contra Godet, 212; Moo, I:345-346). This group is described as “those who did not sin by breaking a command.” [This would be innocent children and infants, etc.] - CPNIV
who is a type [figure; pattern; of Him who was to come -- The "one" who was to come of course refers to Christ, who was yet to come from Adam’s perspective.