The Gospel is the good news about the great salvation accomplished by Jesus Christ. In accomplishing our salvation, Christ both perfectly obeyed the law of God and offered himself as a sacrifice in our place, atoning for our sin, propitiating the just wrath of God toward us, and reconciling God to us.  The Bible says that we are corrupt, sinful, unprofitable, under the wrath and condemnation of God, and utterly without hope, apart from God's sovereign mercy. But God shows his love for us in that while we were still sinners, Christ died for us. The Lord promises that all those who repent and believe in the Lord Jesus Christ will be saved.

Any attempt to make anything but God's mercy the cause of our salvation must be rejected as a dangerous false gospel. We cannot be saved by our good works. For by grace are you saved through faith, and that not of yourselves, it is the gift of God. The ground of our justification is not found in a ceremony, or in the church, or in our education, or in our family, or in anything but Christ's meritorious work received by grace through faith. Instead we must trust in his righteousness to supply the righteousness we do not have. Even our faith is a gift of God, a result of God's love and mercy toward us.

However once we have been saved, we will do good works because God has changed our hearts to desire to do His will.    

The simple truth of the Gospel is repeated over and over in God's word, in passages such as Romans chapter 3:

What then? are we better than they? No, in no wise: for we have before proved both Jews and Gentiles, that they are all under sin;

As it is written, There is none righteous, no, not one:
There is none that understandeth, there is none that seeketh after God.
They are all gone out of the way, they are together become unprofitable;
There is none that doeth good, no, not one.
Their throat is an open sepulchre;
With their tongues they have used deceit;
The poison of asps is under their lips:
Whose mouth is full of cursing and bitterness:
Their feet are swift to shed blood:
Destruction and misery are in their ways:
And the way of peace have they not known:
There is no fear of God before their eyes.

Now we know that what things soever the law saith, it saith to them who are under the law: that every mouth may be stopped, and all the world may become guilty before God. Therefore by the deeds of the law there shall no flesh be justified in his sight: for by the law is the knowledge of sin.

But now the righteousness of God without the law is manifested, being witnessed by the law and the prophets; Even the righteousness of God which is by faith of Jesus Christ unto all and upon all them that believe: for there is no difference: For all have sinned, and come short of the glory of God; Being justified freely by his grace through the redemption that is in Christ Jesus: Whom God hath set forth to be a propitiation through faith in his blood, to declare his righteousness for the remission of sins that are past, through the forbearance of God; To declare, I say, at this time his righteousness: that he might be just, and the justifier of him which believeth in Jesus.

Where is boasting then? It is excluded. By what law? of works? Nay: but by the law of faith. Therefore we conclude that a man is justified by faith without the deeds of the law.

-Romans 3:10-28 - King James Version