Now I would remind you, brothers, of the gospel I preached to you, which you received, in which you stand and by which you are being saved.
The people Paul is writing to in Corinth are Christians - Paul calls them brothers and says that they recieved the gospel. So why is he reminding them of the gospel? He is reminding them of the good news about Jesus life, death and resurrection because it is not just something that they heard and understood in the past, but it is also the thing "by which they are being saved". Their growth in the Christian life doesn't come from moving beyond the gospel but by going deeper into it.
- The way to overcome your habit of lying to cover up your mistakes is to understand that the gospel tells us that, although God knows every "mistake" we've made along with every sinful thought and attitude, He loves us and we are accepted by Him not because we stopped doing things wrong or successfully managed to cover them up but because of Jesus' death on the cross.
- The way to stop reacting in anger to your spouse is to understand how God is slow in anger but rich in love towards us, despite the fact that we constantly provoke Him by trying to take His place.
As Tim Keller says, "the gospel is not the ABC of Christianity, it is the A to Z"