SO THEN AS THROUGH ONE TRANSGRESSION THERE RESULTED CONDEMNATION TO ALL MEN: ara oun os di enos paraptomatos eis pantas anthropous eis katakrima: (Ro 5:12,15,19; 3:19,20)  

Notice in the translations above how the KJV rendering leads to an interpretation slightly different than the other translations (including Young's Literal). Remember translations no matter how literal can have some interpretative bias which is why it is always good to go back to the original languages!

So then (ara oun) is a phrase indicating a conclusion is being drawn. Most commentators also feel that this conclusion section completes the thought that Paul broke off at the end of Romans 5:12.

And so here Paul summarizes the contrast between Adam and Christ. What is the conclusion/summary? There are two results  - in verse 18 all men experience either condemnation (in Adam) or justification (in Christ) and in verse 19 all men are either made sinners (in Adam) or made righteous (in Christ).

A T Robertson notes that this is the

Conclusion of the argument. Cf. Romans 7:3, 7:25; 8:12, etc. Paul resumes the parallel between Adam and Christ begun in Romans 5:12 and interrupted by explanation (Romans 5:13f) and contrast (Romans 5:15; 5:16; 5:17). (Word Pictures in the New Testament)

Guzik observes...

From this passage, Adam and Jesus are sometimes known as the two men. Between them they represent of all humanity, and everyone is identified in either Adam or Jesus. We are born identified with Adam; we may be born again into identification with Jesus.

The idea of Adam and Jesus as two representatives of the human race is sometimes called Federal Theology or Adam and Jesus are sometimes referred to as Federal Heads. This is because under the federal system of government, representatives are chosen and the representative speaks for the people who chose him. Adam speaks for those he represents, and Jesus speaks for His people.

Again, someone may object: “But I never chose to have Adam represent me.” Of course you did! You identified yourself with Adam with the first sin you ever committed. It is absolutely true that we were born into our identification with Adam, but we also choose it with our individual acts of sin. (Romans 5)

James Montgomery Boice has a superb introduction to this summary section...

I do not know when or where it happened, but somebody was sitting in his apartment, getting ready to go to bed, when he heard his neighbor drop a shoe on the floor above him. The upstairs neighbor was obviously getting ready for bed, too, and the man below him waited for the thud of the other shoe. Afterward he must have talked about it, and the expression “waiting for the other shoe to drop” became an expressive figure of speech in our language.

Now we come to what we have been waiting for ever since we started to study Romans 5:12–21. Our expectation arose because Paul began this great passage with a contrast: “Therefore, just as sin entered the world through one man, and death through sin, and in this way death came to all men, because all sinned. …” But just when we were expecting the second half of that thought, he broke it off, and everything we have been studying since has in a sense been a digression, or parenthesis.

In fact, there have been two major digressions, which it might be helpful to review before proceeding.

First, Paul explained the sense in which “all sinned.” He did not mean that all have become sinners and have therefore sinned, though we would naturally think this, but rather that each of us was declared a sinner because of Adam’s original sin or transgression. It is true that we also sin and should be condemned for that, if there were nothing more to be said. But that is not Paul’s meaning. He meant that all have been accounted sinners in Adam, so that those who were going to be saved could be accounted righteous in the Lord Jesus Christ.

Since this digression finished at the end of verse 14, we again expected the other shoe to drop. But instead of completing the contrast introduced by verse 12, Paul worked in another long parenthesis to show the differences between our union with Adam, on the one hand, and our union with Jesus Christ, on the other. This second digression started at verse 15 and occupied the next three verses.

It is only when we get to verse 18 that the second shoe finally falls and we get the full impact of the contrast. Paul backs up to give it, restating the first part again, although in slightly different words: “[1] Consequently, just as the result of one trespass was condemnation for all men, [2] so also the result of one act of righteousness was justification that brings life for all men.”
There we have it!

But then, lest we have fallen asleep in the meantime and have somehow missed the point after this long wait, Paul makes it again in verse 19, adding: “[1] For just as through the disobedience of the one man the many were made sinners, [2] so also through the obedience of the one man the many will be made righteous.” (Boice, J. M. Romans. Grand Rapids, Mich.: Baker Book House)

The 4 major contrasts are seen in tabular form...

At first glance this chart suggests all men will be justified (made righteous) but the context of Romans and the NT clearly indicates the reference is all men who are justified by faith alone. Paul is not teaching universalism or that all men will be justified (saved). Recall that in Romans 5:17 (note) Paul speaks of life for those who receive it. The point is that you don't have to do anything to be condemned. Condemnation is Adam's "gift" to you. But if you want to be justified, you must receive God's free gift by grace through faith.

McGee sums up Romans 5:18 as descriptive of...

the underlying principle of the imputation (act of crediting or laying responsibility upon) of sin and the imputation of righteousness. This is the doctrine of the federal headship of the race in Adam and Christ.

One (1520) (heis) is the cardinal number one and in this case refers to Adam's first transgression in eating from the fruit God clearly commanded he not eat. One transgression is all it took to "infect" the entire human race (for all originate from Adam) with a deadly "virus" called "sin".

One transgression - One man Adam committed one act of selfish disobedience resulting in condemnation for "all men." Now because of Adam's one transgression, all men are born condemned and are all guilty before God, fully deserving the eternal flames of hell. Paul says that the condemnation is universal, coming to "all men" without exception. Apart from Jesus Christ, the whole human race stands condemned by Almighty God.

Condemnation (2631) (katakrima from katá = against, down + kríno = judge and so pronounce sentence against) appears only in Romans 8:1, 5:16; 18. The idea literally is of judgment "coming down" on someone. Paul says God’s judgment is going to come down upon all men because of Adam's sin.

All men...all men - Paul is using all men with two different meanings for the sake of parallelism, a common practice in the Hebrew Old Testament, which is similar Paul's repetition of the phrase the many in Romans 5:15 (note). The first all covers all humanity who are born into Adam. The second all refers to that part of the first all who by grace through faith are reborn into the Last Adam, Christ (Paul repeatedly emphasizes righteousness and faith -  see notes Romans 1:16; 17; 3:22; 3:28; 4:5; 4:13. To reiterate - Paul is not teaching universal salvation.)

EVEN SO THROUGH ONE ACT OF RIGHTEOUSNESS THERE RESULTED JUSTIFICATION OF LIFE TO ALL MEN: houtos kai di enos dikaiomatos eis pantas anthropous eis dikaiosin zoes: (Ro 3:21,22; 2Pet 1:1) (Jn 1:7; 3:26; 12:32; Acts 13:39; 1 Cor 15:22; 1Ti 2:4-6; Heb 2:9; 1Jn 2:20)

One act of righteousness - This refers not to Jesus' perfectly righteous life but to His obedient submission to the Cross. When Christ died, He died for "all men" without exception and without distinction. In some mysterious sense, His death paid the price for the sins. However, the effect of that wondrous death will never be made real in one's life until one personally by a conscious choice receives God's gift by simple faith.

Righteousness  (1345) (dikaioma from dikaióo = to justify <> díkaios = just, righteous <> dike = right) refers to what God has declared to be right.

Vine adds that dikaioma

is rightly rendered “act of righteousness.” It refers to that which Christ accomplished at His death, and stands in contrast to dikaiosune, righteousness simply as a quality.

Justification (1347) (dikaiosis from dike = right, expected behavior or conformity, not according to one’s own standard, but according to an imposed standard with prescribed punishment for nonconformity) denotes the act of pronouncing righteous -- justification, acquittal. Dikaiosis in this passage refers to justification which results in life, the life He gives us through His resurrection and the sharing of His life with us.

Regarding the somewhat difficult to understand phrase justification of life Kenneth Wuest writes that...

The words of life are genitive of description in the Greek text, describing the quality of the righteousness bestowed upon man. It is a righteousness which is connected with the impartation of spiritual life. In itself, this righteous standing is a purely legal matter and does not impart life nor change character. But it is accompanied by the life that God is, imparted to the believing sinner in regeneration. (Wuest, K. S. Wuest's Word Studies from the Greek New Testament: Eerdmans or Logos)

All men does not mean all men will be saved, because Scripture amply attests to the truth that salvation is only for those who exercise faith in Jesus Christ (see notes Romans 1:16-17, 3:22, 3:28, 4:5, 4:13; cp similar phrase the many in Romans 5:15 [note])

><> ><> ><>

Our Daily Bread - Through one Man's righteous act the free gift came to all men, resulting in justification of life (Romans 5:18).

At noon on January 1, 1863, Abraham Lincoln received the final draft of the Emancipation Proclamation. Twice the president picked up his pen to sign it, and twice he laid it down. Turning to Secretary of State William Seward, he said, "I have been shaking hands since 9:00 this morning, and my right arm is almost paralyzed. If my name ever goes into history, it will be for this act, and my whole soul is in it. If my hand trembles when I sign the proclamation, all who examine the document hereafter will say, `He hesitated.— The president then took up the pen again and slowly but firmly wrote, "Abraham Lincoln." That historic act endeared Lincoln to the world as the Great Emancipator.

One greater than Lincoln and with even surer resolve brought free­dom to the human race. Jesus signed our liberty with His own blood by dying on the cross to release us from the awful slavery of sin. Oswald Chambers wrote, "Never tolerate the idea of martyrdom about the cross of Jesus Christ. The cross was a superb triumph in which the foundations of hell were shaken. [Jesus Christ]...made the redemp­tion the basis of human life, that is, He made a way for every son of man to get into communion with God."

Having trusted the Savior, we are free from sin's condemnation. By His Spirit we have the power to turn from sin and live for Him. And doing so is the only way to honor Christ—our Great Emancipator.—D J De Haan (Our Daily Bread, Copyright RBC Ministries, Grand Rapids, MI. Reprinted by permission. All rights reserved)

FOR AS THROUGH THE ONE MAN'S DISOBEDIENCE  THE MANY WERE MADE (constituted, declared) SINNERS: hosper gar dia tes parakoes tou enos anthropou hamartoloi katestathesan (3PAPI) oi polloi: (Isa 53:10-12; Da 9:24; 2Cor 5:21; Eph 1:6; Rev 7:9-17)

For (gar) emphasizes that this verse is an explanation of the former verse and not a mere repetition. But you may be asking what would be confusing about what he has just stated? Why does it need an explanation?"

Barnes addresses this question noting that...

By the former statements it might perhaps be inferred that men were condemned without any guilt or blame of theirs. The apostle in this verse guards against this, and affirms that they are in fact sinners. He affirms that those who are sinners are condemned, and that the sufferings brought in, on account of the sin of Adam, are introduced because many were made sinners. Calvin says, "Lest any one should arrogate to himself innocence, [the apostle] adds, that each one is condemned because he is a sinner." (Albert Barnes. Barnes NT Commentary)

Newell notes that...

There is no more direct statement in Scripture concerning justification than we find in Romans 5:19.

Again Paul refers to Adam's disobedience in the Garden of Eden.