The central belief of Christianity is the Gospel of Jesus Christ.
Jesus began His earthly ministry "preaching the Gospel of God, and saying, 'The time is fulfilled, and the kingdom of God is at hand; repent and believe in the gospel.'" (Mark 1:14b-15) He concluded it with the command to the church to "go into all the world and preach the Gospel to all creation." (Mark 16:15b; Matt 28:18-20)
But what is the Gospel?
The word "gospel" literally means "good news," thus it has to be at least three things. First, it has to be news. Second, it has to be good. Finally it has to be about Jesus Christ.
The Gospel is news, therefore it is not something we do, but something that has been done that we learn about.
The Gospel is good news, therefore this news that we hear should be sweet to the ears and warm our hearts.
The Gospel is good news about Jesus Christ, therefore the good news we hear and learn is not about ourselves, but about who and what Jesus Christ is and has done.
To believe in the good news, however, one has to understand the full nature of the problem that it addresses. The good news that the Second World War was over, and that the Allies had won, is meaningless to an ardent Nazi let alone to the delusional person who thought there was no war.
The Bible teaches, and reality confirms, that all humanity by its very nature is fallen and sinful (Eph 2:1-3; Rom 1:18-3:23). We think and act contrary to the Law of God and thus bring about His just wrath and our condemnation.
Question: do you keep the Ten Commandments in both deed and thought? Can you even name them (Ex 20:1-17; Matt 5:21-48)? If we are honest with ourselves we do not deserve God's mercy, but His judgment. For if God is good and holy then He must punish the wicked, which is everyone.
You might be saying, "This doesn't sound like good news," and you would be right. God's Law, though holy and good, only brings righteous condemnation (Gal 3:10-29). Again, to understand the good news which Christianity proclaims you have to understand the problem. Also, if anyone tells you the Gospel is "do more, try harder," then by definition that isn't good news.
So what is the good news?
The good news of Christianity is that Jesus Christ has taken upon Himself the just wrath of God that condemns us. That is why Jesus died on the cross, that ALL our sins should be put upon Him. Yet, because Jesus is fully God, fully man and is without sin, His death perfectly appeases God's anger and we are declared at peace with God and now are His beloved children because ALL our sins (past, present and future) are completely and totally forgiven in Christ (John 6:35-40; Rom 3:21-31; Eph 2:1-10; Titus 3:3-8).
Can you think of any better news?
This good news, this Gospel, is the message the church is called to proclaim and teach to the world. That Christ has done it all, and we are called to repent (turn from the fallen world and our sins) and believe and trust what Jesus has done and that His work perfectly saves us. So Jesus' Gospel is the very power of God, because it brings all of God's mercy and love perfectly, since it is accomplished by God alone (Acts 2:33-38; 10:36; 13:38-39; Rom 1:16-17; Eph 2:17).
Please note, this faith in the Gospel is not based on our emotional state, but whether we believe in the news of what Jesus has done and trust that He has accomplished what He has promised.
Christ's church is called to proclaim and teach this good news to everyone, both to non-Christians and to Christians alike. The church tells the Gospel to the world because the good news of Jesus Christ alone brings new life and forgiveness. The Gospel is also for Christians, found primarily in the worship on the Lord's Day (i.e. Sunday) through biblical preaching and the Sacraments, because the more we understand and believe this good news the more it will encourage us to love God, live holy lives and to perform good works of loving our neighbor.
When the Gospel becomes the center of the church's life then all things are possible. That is why the Apostle Paul can write in prison just before his death, "Therefore do not be ashamed of the testimony of our Lord or of me His prisoner, but join with me in suffering for the gospel according to the power of God, who has saved us and called us with a holy calling, not according to our works, but according to His own purpose and grace which was granted us in Christ Jesus from all eternity, but now has been revealed by the appearing of our Savior Christ Jesus, who abolished death and brought life and immortality to light through the gospel." (2Tim 1:8-10)