The gospel is not behavior modification, becoming a better person or learning to become more moral. It is not taking the life of Jesus as a model way to live or transforming/redeeming the secular realm. It is not living highly communal lives with others and sharing generously in communities who practice the way of Jesus in local culture. These may all be good things but they are not to be confused with the gospel. They may be the fruit of the gospel, and they will surely accompany the gospel, and while God may use them as means to authenticate the gospel and make our proclamation of the gospel more fertile in hardened hearts, yet they are not to be viewed as replacements for the gospel.

Did you notice the one characteristic of all of the above activities has nothing to do with what Christ has done for us, but all about what we do for him. The true gospel, rather, is news about what Christ the Saviour, has already done for us (in his life, death and resurrection) rather than instruction and advice about what you are to do for God. Christ's accomplishment, not ours, is the essence of the gospel. Above all, the gospel of Christ brings good news, rather than instruction about our behavior. The gospel of not about what we do, but our acts inevitably spring up and overflow in thanksgiving due to what Christ has done for us.

In short, the Gospel is the life-altering news that Jesus Christ, the eternal Son of God, became man, lived a sinless life under the Law, died for sinners and rose again to reconcile them to himself, eternally victorious over every enemy that stood between God and man. Now, because of this redemptive work, there is nothing that separates those who believe from their Creator and all the benefits that He promises in him. D.A. Carson says the gospel centers "upon Jesus Christ and what God has done through him. The essential points of the gospel are Jesus Christ's status as the Son of God, his genuine humanity, his death for our sins, his burial, resurrection, subsequent appearances, and future coming in judgment. That no one is justified but in the gracious work of Jesus Christ in his death and resurrection. It is not merely a recital of theological truths and historical events; rather, it relates these truths and events to situations of every individual believer."

But in order to fully understand what the Gospel is, it is important to understand why the Gospel is needed.

It is helpful to see the gospel in the context of human history starting with God's creation of all things, man's rebellion against the Creator, his subsequent fall into corruption and God's redemption of that which was lost. Adam, the first man, had the capacity to do every good work the law required; which men, since the fall, have not. Having fallen headlong into sin, God cursed Adam with death (Gen 2:17, 3:19-22), and with the removal of His Spirit (1 Cor 2:14), a penalty he passed on to all his posterity. Man squandered his stewardship and put himself in the position of a moral debt he cannot repay. Now mankind's spiritually bankrupt condition and fallen nature, which is beyond repair, render it necessary that if he is to be restored, the help will have to come from the outside. That redemption comes from God and comes in the form of the gospel. This gospel is not something man made up or a well-informed opinion, but is good news directly revealed from Almighty God regarding what He has done in Jesus Christ to rescue all those who have called on His name. Yes, it is a divine rescue, a complete deliverance ... not advice, not a moral improvement program, nor a philosophy of life, since we need sovereign mercy, not assistance. The proud, or those who fail to see their moral impotence to save themselves, will reject this gospel. But this is GOOD NEWS to the poor and broken hearted, (the spiritual bankrupt who have lost all confidence in their own efforts) ... So all you poor, broken sinners, abandon despair and banish your laments because of what God has done in His Son, Jesus Christ the Messiah to deliver His people from their sins.

I once heard it said that there are two religions in the world: 1) human attainment and 2) Divine accomplishment. Lets consider the first one; human attainment, which is the natural inclination of us all. In His Law, God calls us to perfect obedience to His holy commands, yet an honest assessment of ourselves will force us to acknowledge that we all fall woefully short of doing so, leaving no hope in ourselves. But in the Gospel, Jesus mercifully obeys the commands for us. Christ’s full obedience to all the prescriptions of the divine law…and His willing obedience in bearing all the sanctions imposed for our disobedience to that law is both the ground of God’s justification of sinners like us and makes available a perfect righteousness that is imputed or reckoned to those who put their trust in him. In other words, The gospel is not about any merit I have, but is based upon Jesus' Person and merit alone. It is not what we have done for Jesus, but what Jesus has done for us (Rom 5:19, 2 Cor 5:21, Phil 2:8). Where Adam failed, Jesus prevailed. It is God's promise to us, not our ability to keep our promise to Him. In the covenant rainbow sign with Noah, God says He "remembers" never to flood the world again, so likewise in the covenant in Christ's blood, God "remembers" not to treat us as we justly deserve for our sins. The mystery of God has been made manifest in the Person and work of the Son, who, in his wrath absorbing sacrifice, frees the prisoners, gives sight to the blind, breaks loose the chains and changes hearts of stone into hearts of flesh. We were once taken captive to do Satan's will and could not escape using our own resources, but Christ has set us free. Christ, in His cross work, does for us what we could not do for ourselves. He lived the perfect life that we should have lived and died the death we should have died, in order to free us so that we might then proclaim His excellencies, make known his gospel and spread justice and mercy to the poor.

Dr. Tim Keller once said "...the gospel is news about what God has already done for you, rather than instruction and advice about what you are to do for God... In other religions, God reveals to us how we can find or achieve salvation. In Christianity, God achieves salvation for us. The gospel brings news primarily, rather than instruction. " ...the gospel is all about historic events, and thus it has a public character. "It identifies Christian faith as news that has significance for all people, indeed for the whole world, not merely as esoteric understanding or insight." [Brownson, p. 46] ...if Jesus is not risen from the dead, Christianity does not "work". The gospel is that Jesus died and rose for us. If the historic events of his life did not happen, then Christianity does not "work" for the good news is that God has entered the human "now" (history) with the life of the world to come....the gospel is news about what God has done in history to save us, rather than advice about what we must do to reach God. The gospel is news that Jesus' life, death, and resurrection in history has achieved our salvation...Jesus does not just bring good news; he is the good news."

There is no salvation outside of the Lord Jesus Christ. So trust in Christ and not in your own righteousness. But some refuse the free gift of God because they trust in their own goodness. As the Puritan Thomas Watson once said:

[Some people think] ...they are so good, that they scorn God's offer of mercy. Indeed these are often in the worst condition: these are they who think they need no repentance (Luke 15:7). Their morality undoes them. They make a "savior" of it, and so on this rock they suffer shipwreck. Morality shoots short of heaven. It is only nature refined. A moral man is but old Adam dressed in fine clothes. The king's image counterfeited and stamped upon brass will not go current. The moral person seems to have the image of God—but he is only brass metal, which will never pass for current. Morality is insufficient for salvation. Though the life is moralized, the lust may be unmortified. The heart may be full of pride and atheism. Under the fair leaves of a tree, there may be a worm. I am not saying, repent that you are moral—but that you are no more than moral. Satan entered into the house that had just been swept and garnished (Luke 11:26). This is the emblem of a moral man, who is swept by civility and garnished with common gifts—but is not washed by true repentance. The unclean spirit enters into such a one. If morality were sufficient to salvation, Christ need not have died. The moral man has a fair lamp—but it lacks the oil of grace."

Jesus is Lord and creator - the only rightful king of all creation ... king of all things both seen and unseen. To those who worship the false idols of their hearts (any God-replacement) take heed ... Jesus will soon be invading with His armies and will overthrow his enemies and all injustice with the breath of His mouth. But He is offering pardon in advance of His invasion to all those who receive Him (John 1:12, 13). Those who have joined themselves to Him now before He invades will be considered His ally and He will raise them up to be co-heirs with Christ as sons. The alternative is to be under the wrath of the king. We herald this announcement: that the True King is on the throne and he'll be invading. The gospel is not merely an invitation it is a command to all those going their own ways. Will you heed the command? Jesus is Lord, repent and believe."(Bill Wilder) But because of the blindness sin has cast over us, Jesus says, no one can believe in Him unless the Father grants it through the regenerating work of the Holy Spirit (John 6:63-65). So those who, by the grace of God, trust in Jesus and His work can be assured, on the sure testimony of Scripture, that their sins are forgiven and have the promise of God: eternal life.

To Summarize:
Man was created to glorify God & Enjoy Him forever
"Worthy are you, our Lord and our God to receive glory and honor and power, for You created all things." (Rev 4:11) "Do all to the glory of God" (1 Cor 10:31)

Man has failed to glorify God & is under His just condemnation
"For all have sinned..." (Rom 3:23) The wages of sin is death (Rom 6:23) "These will pay the penalty of eternal destruction" (2 Thes 1:9)

Jesus fully bore the wrath and suffered the punishment sinners deserve
Not wishing that sinners perish forever, God determined to save a people for Himself in the Eternal Son who became a man and lived the life we should have lived and died the death we justly deserve. God loves sinners and sent His Son to be the wrath absorbing sacrifice for their sin (1 John 4:10; John 6:37) he "...gave His life as a ransom for many" (Mk 10:45) & "rose again" from the dead (2 Cor 5:15) on their behalf.

All who, by the grace of God, turn to Jesus in submissive faith are forgiven
If you confess you are a sinner in need of Christ then God has begun to work in you a life-changing, eternally satisfying relationship with Himself! "Repent and believe the gospel (Mk 1:5) "In Your presence is fullness of Joy (Ps 16:11). So leave your self-righteousness, and your sins. Fly unto the Lord Jesus Christ, and receive his righteousness to be your covering, and his blood to be your atonement. If your trust is in Jesus alone for your salvation (that is, if you have no hope save for Christ's mercy alone) then you can be assured that your sins are forgiven and He has granted you eternal life.

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For Further Study
The Gospel Of Jesus Christ by D.A. Carson - 1 Corinthians 15:1-19 (.pdf)
The Everlasting Righteousness by Horatius Bonar outstanding!
Paul's Definition of a Christian by John Hendryx
Repenting of our Good Works by John Hendryx
Difficult Questions, Certain Answers (.pdf) @Monergism - A redemptive historical gospel tract
The Sum of Saving Knowledge Westminster Assembly Supplemental Document 
All of Grace by C.H. Spurgeon