Martyn Lloyd-Jones tells the story of how the church he grew up in began with a great revival, but slowly died over time. When he inquired about what had led to this decline, an older man attributed it to the fact that when the revival had taken place, the gospel was being heralded regularly and powerfully, but over time it became assumed that such gospel proclamation was no longer necessary, since those who attended the church were already Christians.
Lloyd-Jones determined that he would always preach the gospel, no matter who he was preaching to. Not only did he consider it a “fatal assumption” to think that just because someone attends church, they must be a Christian, he also believed that Christians never outgrow the need to hear the gospel. 
The gospel is not good advice about what you ought to do for God, it’s the good news about what God has done for you in Christ. Paul says that the gospel is “the power of God for salvation to all who believe” (Romans 1:16). Paul told the Ephesians that it was when they heard the gospel of their salvation and believed in Jesus that they were saved (Ephesians 1:13)
Clearly those who do not yet believe need to hear the gospel, so they can know who Jesus is and what he has done for them, so that they can believe and be saved. What about those who already believe; what do they need? Biblical instruction? Absolutely. But do you know what else believers need in order to grow in their faith and relationship with God? They need to hear the gospel.
The Gospel is Not Just the Starting Point of Christianity, It is the Beating Heart of Christianity
In his letter to the Galatians, Paul wrote to a group of Christians who, even though they were committed followers of Jesus, they were still trying to be justified before God by their own works. Paul wrote to these believers to remind them of the gospel and instruct them about the gospel: what Jesus had accomplished for them, and what it meant for their lives.
Are you so foolish? Having begun by the Spirit, are you now being perfected by the flesh? (Galatians 3:3)
Even though they were already believers, they still needed to be hear the gospel.
An Apostolic Pattern
This is not unique to Paul’s letter to the Galatians; it is a pattern that is seen throughout the apostolic letters in the New Testament. When the apostles wrote to the early Christians, they did not merely tell them how they ought to live now that they were followers of Jesus, rather they reminded them of the gospel, and encouraged them to respond to the gospel in every area of their lives.
The apostles’ pattern was to remind believers of the gospel, as the motivation and the pattern for the Christian life.
What this means is that you never outgrow the gospel. No matter how long someone has been a Christian, they will never get to the place where they no longer need to hear the gospel.
It means that the gospel is not just the means by which people become Christians, it is also the means by which we grow as Christians, as we believe, embrace and apply the gospel to every area of our lives.
When Paul instructed the Ephesians about marriage, he didn’t tell husbands and wives to love and respect each other because it is “the right thing to do,” rather he instructed them about marriage on the basis of the gospel (Ephesians 5:22-33)
When Paul wrote the Corinthians about generosity, he didn’t tell them that this is what they have to do because they are Christians, rather he appealed to them on the basis of the gospel, saying, “For you know the grace of our Lord Jesus Christ, that though he was rich, yet for your sake he became poor, so that you by his poverty might become rich.” (2 Corinthians 8:9)
This is motivation on the basis of the gospel of God’s grace. Whereas laws can control behavior, they do not affect the heart. Conversely, when the heart is changed by the love and grace of God, actions will follow.
The apostolic pattern in the New Testament is to preach the gospel both to unbelievers and believers, and to show how the gospel speaks to every area of life. May we be those who follow this pattern by applying the gospel to all of life, and faithfully proclaiming it whenever we teach or preach, no matter who our audience.