Is the Gospel for believers?

The Gospel is the reason why we don't turn our tables against God after being gratuitously saved by faith and grace. Turning our tables against Him means reversing the terms on how sanctification happens - by determining our walk with Him and our transformation on our own terms, by our own effort. We fall time and again, but guilt has no hold over us; obedience to God is inseparable from love - it's not sterile duty because it's a response to the Gospel -the power of God that proves Him to be loving, just, wise and ever so majestic- because the Gospel applies to Christians every single day.

On sanctification, Jerry Bridges writes,

When our sense of guilt is taken away because our consciences are cleansed by the blood of Christ, we are freed up to love Him with all our ears and souls and minds. In fact, not only are we freed up, we are motivated in a positive sense to love Hm in this wholehearted way. Our love will be spontaneous in an outpouring of gratitude to Him and fervent desire to obey Him.

Jesus said, "He who has been forgiven little loves little" (Luke 7:47). In the context of that statement He essentially said the converse is also true: Those who have been forgiven much love much. Therefore, we can say that the extent to which we realize the total forgiveness and cleansing from those sins, will determine the measure of our love to God.

So if we want to grow in our love for God and in the acceptable obedience that flows out of that love, we must keep coming back to the Cross and the cleansing blood of Jesus Christ. That is why it is so important that we keep the gospel before us every day. Because we sin every day, and our consciences condemn us every day (though Christ has removed sin's guilt and reign in our lives), we need the gospel every day.

The Discipline of Grace - God's Role and Our Role in the Pursuit of Holiness (1994; parenthesis mine)