It is good for Christians to regularly rehearse the gospel to themselves and to others. It is in the gospel that we are saved. It is through the gospel that the church is built up. It is in the gospel that God is seen in his greatness and glory in the world. It is through the gospel that false views of God will be squashed. It is in the gospel that Christian growth and strength for Christian living come.
Ligon Duncan gives an excellent summary of the gospel below.
Here is John Piper on the same gospel as being for Christians as well as non-Christians:
As Christians, our regular speaking of the testimony of Jesus is a means by which God is working powerfully in the world. Consider the striking statement about the power of a gospel testimony in Revelation 12:
“And I heard a loud voice in heaven, saying, ‘Now the salvation and the power and the kingdom of our God and the authority of his Christ have come, for the accuser of our brothers has been thrown down, who accuses them day and night before our God. And they have conquered him by the blood of the Lamb and by the word of their testimony, for they loved not their lives even unto death.” (Rev. 12:10-11)
These saints described in Revelation were not ashamed of the gospel and they were the mouthpieces of God in the world. Their witness was strong and effective. The text says that they overcame “by the blood of the Lamb,” but also by “the word of their testimony.” In the same way, the Church triumphs in every circumstance and every trial as its members are vocal witnesses of the eternal gospel.
There is, then, a responsibility for Christians, not only to preach the gospel, but to preach the right gospel. We have a responsibility to get the gospel right. We ought to always be growing in our understanding of, and ability to articulate, the gospel. We ought to be growing in our understanding of the doctrines of the gospel. No two explanations of the gospel will be the exact same but all true explanations of the gospel will have certain similarities. They involve the same truths about a holy God, sinful man, Christ’s saving work, and our response to him. As you continue to grow in your understanding and appreciation for the gospel, here are some questions that you may want to consider asking and answering.
Who is God and what is he like?
What is the problem in my relationship to him?
What has God done in Christ to remedy that problem?
What does God say my response to him should be?