The Gospel is for believers as well

The apostle Paul wrote the letter of Romans to the Christians in Rome.

Being a Roman citizen by birth, but having never been there, Paul was eager to finally see Rome. He was planning a new mission to Spain (Romans 15:24), so Paul considered it the perfect opportunity to see Rome enroute to Spain.

Whilst there, Paul would also seek to be strengthened by the Christians there in Rome and likewise, he could also help them in their faith

Romans 1:11-12:

"For I long to see you so that I may impart some spiritual gift to you, that you may be established; that is, that I may be encouraged together with you while among you, each of us by the other's faith, both yours and mine."

Not only did Paul want to encourage them in their faith as well as be encouraged by them, Paul wanted to preach to them the gospel.

Romans 1:15:

"So, for my part, I am eager to preach the gospel to you also who are in Rome."

So the original recipients of Paul's letter were the members of the church in Rome. The majority were both Jewish and Gentile Christians.

The book of Romans has some of the greatest summaries of the gospel contained within it.
Martin Luther, the great reformer said of Romans:

"This letter is truly the most important piece in the New Testament. It is purest Gospel..."

The interesting question here is, why does Paul preach the gospel to Christians?
Look at these verses:

Paul preached the gospel to the saints at Ephesus:

Ephesians 1:3-8:
“Blessed be the God and Father of our Lord Jesus Christ, who has blessed us with every spiritual blessing in the heavenly places in Christ,
just as He chose us in Him before the foundation of the world, that we would be holy and blameless before Him In love
He predestined us to adoption as sons through Jesus Christ to Himself, according to the kind intention of His will,
to the praise of the glory of His grace, which He freely bestowed on us in the Beloved.
In Him we have redemption through His blood, the forgiveness of our trespasses, according to the riches of His grace which He lavished on us. In all wisdom and insight.”

He also preached the gospel to the church at Corinth:

1 Corinthians 15:1-2 says:
“Now I make known to you, brethren, the gospel which I preached to you, which also you received, in which also you stand,
by which also you are saved, if you hold fast the word which I preached to you, unless you believed in vain.”

Note here in these verses, Paul points out to the Corinthians, that the gospel is:
1) by which also you are saved
2) in which they now stand

We all know the most obvious fact regarding the gospel, that is, it is for the unsaved in which they can gain salvation. But the fact is, Scripture not only preaches justification by faith and the redemptive work of Christ at the cross for the unsaved, but it also preaches the gospel to believers even far more than it does to those who are not saved.

I mean, just look at the Pauline epistles – they are letters for the Christian churches that outline instructions for Godly and righteous living.

The gospel, I feel, has been consigned to only a message of good news for the unsaved and that’s about it.
But this would be a grave mistake on our part.
The gospel is not something we experience at the point of salvation and then shelve for some time and then pull out when you encounter those who don’t know Christ.

This attitude shows a lack of understanding, on our part of the true Christian life and of salvation by grace, through faith in Christ.
We, at times, forget the fact that while we were (and will be) passive participants in our justification and future glorification, we are still very much participants in our on-going sanctification, the process in which we become more like Christ, while the hold of sin on us lessens.

So, we should regularly remind ourselves of the gospel and the power that is gained within.
Why? Well, living a true Christian life and following in the footsteps of Christ is inextricably connected to the work of Christ at the cross.

Because of the gospel, once we were dead, now we have life. 
Because of the gospel, our former slavery to sin and the guilt of sin is broken.
Because of the gospel we can now escape the temptation of sin through the strength which He provides.
Because of the gospel, when we sin, we are forgiven.
Because of the gospel, our prayers for are heard. 
Because of the gospel, we can worship our awesome God.
Because of the gospel, we never need to feel that we are on a constant performance treadmill and in which the Christian life seems an impossible standard of living.
Because if the gospel, we can rest knowing that God will never hold our sin against us.

Really good News
The gospel, then really is good news. It is especially good news for Christians.
We should dwell in the gospel more often, remind ourselves daily of how amazing the gospel really is and experience the life-transforming and sanctifying power if it.