Romans 5:15-21

Rom 5:15 (KJV)
But not as the offence, so also is the free gift. For if through the offence of one many be dead, much more the grace of God, and the gift by grace, which is by one man, Jesus Christ, hath abounded unto many.

Now Paul contrasts the offence and the free gift of Grace. He states that it is through the offence of the first Adam, the entire human race was plunged into sin which resulted in not only physical death but spiritual death to those who are not Elect. But the gift of Free Grace came through the Lord Jesus Christ and as a result, those who are the chosen of God will receive the free justification and salvation. The word “abounded” in the Greek carries with it the meaning of “to be superior” or “excel in.” This means that the Free Grace which Christ gives His Elect is abounding much more than the offense which plunged the entire human race into sin and death.

Rom 5:16 (KJV)
And not as it was by one that sinned, so is the gift: for the judgment was by one to condemnation, but the free gift is of many offences unto justification.

Paul continues the contrast between the offence and the gift of Grace. The human race was plunged into sin which was the judgment of God upon the human race because of Adam’s sin. That judgment eventually led and will lead to the eternal condemnation of all the unsaved. Contrasting the judgment of the human race is the fact that the free gift of Grace will justify all the Elect of God and it will be as if they never sinned since their souls will be made ready for Heaven at the moment of salvation since justification is a one time event.

Rom 5:17 (KJV)
For if by one man's offence death reigned by one; much more they which receive abundance of grace and of the gift of righteousness shall reign in life by one, Jesus Christ.)

Death did not only reign from Adam to Moses, it reigns until the last day when God will send forth His angels to bring home His Elect. The contrast is that death reigns over the entire population of the world but life reigns in the lives of the Elect of God, even though the Elect of God face physical death, they do not face eternal death, which is the ultimate penalty for the sin of Adam. The Elect of God, upon physical death are ushered into the physical presence of the Lord eternally.
(2 Cor 5:8 KJV) We are confident, I say, and willing rather to be absent from the body, and to be present with the Lord. Death, hell, sin and sickness will never again threaten the believer.

Rom 5:18 (KJV)
Therefore as by the offence of one judgment came upon all men to condemnation; even so by the righteousness of one the free gift came upon all men unto justification of life.

Here is one of those verses which must be carefully looked at. The sin of Adam brought the entire human race under the condemnation of God.
(John 3:18 KJV) He that believeth on him is not condemned: but he that believeth not is condemned already, because he hath not believed in the name of the only begotten Son of God. Contrasting the consequences of the sin of Adam as permeating the entire human race, the free gift of Grace which brings full justification to the believer, is only permeating the body of Christ. The entire population of the world does not receive this Grace but only those whom God has chosen from the foundation of the world. (Rev 13:8 KJV) And all that dwell upon the earth shall worship him, whose names are not written in the book of life of the Lamb slain from the foundation of the world. The free Grace of God brings the believer from the point of death to life eternal. (1 John 5:12 KJV) He that hath the Son hath life; and he that hath not the Son of God hath not life.

Rom 5:19 (KJV)
For as by one man's disobedience many were made sinners, so by the obedience of one shall many be made righteous.

So by the consequential actions of the first Adam, the entire human race was plunged into sin and death but by the obedience of Christ who went to the cross to pay for the sins of His Elect, many have been made and many shall be made righteous, in other words, they have become saved.
(Mat 25:46 KJV) And these shall go away into everlasting punishment: but the righteous into life eternal.

Rom 5:20 (KJV)
Moreover the law entered, that the offence might abound. But where sin abounded, grace did much more abound:

When the written law of Moses came on the scene, the offence of Adam was heightened because the law now showed people that they were sinners. One needs to just look at the Ten Commandments and you will see that it is literally impossible to keep them. The law showed us that we were vile sinners and it showed us how the one sin of Adam, which was disobedience, has now mushroomed into many different types of sin, all with a condemnatory nature. However, Paul contrasts sin and Grace, he shows us that even though the sins of men are heinous, the Grace of God abounds. The word “abound” in the Greek carries with it the meaning of “super abounding, overflowing, or running over.” The Grace of God is so potent that it can quash and remove the sins of those whom God plans to save. Sin can never excel above the grace of God.
(1 John 3:8 KJV) He that committeth sin is of the devil; for the devil sinneth from the beginning. For this purpose the Son of God was manifested, that he might destroy the works of the devil. The works of the devil is sin and the sacrifice of Christ has destroyed sin.

Rom 5:21 (KJV)
That as sin hath reigned unto death, even so might grace reign through righteousness unto eternal life by Jesus Christ our Lord.

Paul now completes his contrast between sin and Grace. Sin can only reign up to a point. Sin cannot reign beyond the grave in the believer’s life. As a matter of fact, sin does not reign beyond the grave in the life of an unbeliever. When an unbeliever is in hell, sin is not reigning but the penalty for those sins are being paid. Sin is committed only on this side of the grave but on the other side, is the condemnation for those sins, which is eternal. This verse contrasts eternal damnation with the fact that Grace will reign in the life of the true believer for eternity but that Grace comes only through the Lord Jesus Christ and never by any acts or works of self-righteousness or rituals.