The difference between God's gospel and man's could not be greater.
In Paul's letter to the churches of Galatia, he is adamant that his gospel was given to him by God himself. As such, his gospel was the authoritative and authenticated Word of Promise entrusted to him by the Word incarnate. This, of course, was in contrast to the "other gospels" that were vying for the attention and acclaim of the Galatians. Paul's passion, which is so often on display in the letter to the Galatians, is indicative of his regard for the gospel of God, which he received "through a revelation of Jesus Christ" (Gal. 1:12). He knew by experience the profundity of the grace and peace that are gifted to sinners by God through "the Lord Jesus Christ, who gave himself for [all] our sins to deliver us from the present evil age" (Gal. 1:3–4). This is the announcement that lies at the heart of the gospel that Paul was commissioned to preach.
But what about those "other gospels"? What were they about? What made them such "accursed" announcements? (Gal. 1:8–9). For one, of course, they emerged from the heart of man, as Paul explains, "For I would have you know, brothers, that the gospel that was preached by me is not man's gospel" (Gal. 1:11, emphasis mine). Again, Paul's gospel was received, not contrived or conjured up. The gospel of God is a message delivered to mankind both because it reports God's doings and dealings with humanity and because men could never have generated such an ingenious way to make sinners holy — namely, by grace. Man's method involves work, effort, and strenuous religiosity. Man's gospel is a motivational message of "Do more" and "Try harder" to span the chasm of separation between God and creation.
The gospel of God is a message delivered to mankind both because it reports God's doings and dealings with humanity and because men could never have generated such an ingenious way to make sinners holy
The following paragraph from Horatius Bonar's excellent treatise Man: His Religion and His World excellently expresses the concept of "man's gospel":
Man tries gradually to lessen the distance between himself and God by his own doings; God annihilates it at once by the doings of another, which is accomplished in a moment that which a whole eternity of doings could not have enabled man to effect. Man plies every effort to crumble into pieces and carry away the mountain barrier that rose between God and him; God steps in and sweeps away the barrier and levels the mountain into a plain. Man tries, by endless installments, to pay the eternal debt that has cast him into prison and made him an alien from his Creator; God comes forth and, in one sum, pays the infinite debt, and the prisoner goes free (p. 50–51).
The difference between God's gospel and man's could not be greater. They are polar opposites. Whereas mankind scratches and claws to ascend some imaginary staircase of righteousness, God in Christ descends at once to make men righteous by giving them his righteousness. Like an incendiary device, the gospel of God explodes any theory that humanity can "do" their way into a favorable or right standing with God the Father. Instead, Jesus comes to do that for them. He comes down and "steps in" to "sweep away the barrier" that precludes our fellowship with our Creator. The Christ of God comes to make peace for us and reconcile us by succumbing to emaciation and crucifixion, "thereby killing the hostility" that alienates us from the Father (Eph. 2:12, 16). In so doing, he makes it possible for sinners to be brought close by his blood (Eph. 2:13).
The report of the gospel, therefore, concerns all the glorious particulars by which this closeness is feasible — namely, through the death and resurrection of Christ. Indeed, preaching the gospel is all about showcasing the divine contours of grace and redemption that God has been writing throughout history and time. It's announcing the inconceivable but decisive message of sin's defeat in the crucified and risen Christ, who exhausts the penalty of sin that weighed the world down. "The exhaustion of the penalty," Bonar continues, "is indispensable. Any other way would be connivance at sin, for you cannot annihilate sin" (51). Man's gospel is a non-starter since it relies upon the capability and competency of broken vessels, a.k.a. human beings, to rectify the problem of sin. For those whose sins are engraved with a diamond-pointed stylus (Jer. 17:1), the hope of "man's gospel" is flimsy, to say the least.
Accordingly, there is only one option and one hope for sinful humans like us, which, as Horatius Bonar concludes, is all about the person and work of Jesus:
The exhaustion of the penalty, then, by the Infinite One in person, is the only way whereby the indelible may be erased, and sin annihilated. This exhaustion of the penalty on behalf of His Church is the work of the Son of God. It was He who passed on into the eternity to come, and gathering together the punishment spread out over the endless ages took it all upon himself, and bore it, till, by exhausting it, he had blotted it out of being . . . This is redemption . . . In no other way could He be both gracious and righteous. But the work has been done. God has reconciled His love and His holiness. And it is this wondrous reconciliation that is made known by the gospel to the children of men. 'A just God, yet a Saviour'; 'just, yet the justifier of the ungodly' — this is the heavenly message. It was on the cross that the reconciliation took place. It was there that the curse was exhausted. And it is to that cross that God is pointing man's eye, that, seeing the great deed done there, he may understand the forgiveness that there is with God, and feel assured that the greatness of his guilt can no longer be a reason why righteousness should refuse him a welcome (pgs. 52–54).
The gospel of God is the word of redemption that comes from outside of us. It is promised by God and carried out by his Word made flesh for us. We could never compose such a message, let alone orchestrate it. Therefore, any concept of "another gospel" is impotent from the outset. Only the gospel of God, which reveals the grace of God in the person of God's Son, possesses "the power of God for salvation to everyone who believes" (Rom. 1:16). Hope for sinners, then, notwithstanding the pile of sins they've accumulated, is found in the definitive word of God's gospel, which gestures to sinners to behold and believe in the work of God on their behalf by which they are made righteous. This is the best news of all.
