"I once assumed (along with the vast majority of professing Christians) that the gospel was simply what non-Christians must believe in order to be saved, while afterward we advance to deeper theological waters. But I’ve come to realize that once God rescues sinners, his plan isn’t to steer them beyond the gospel, but to move them more deeply into it. The gospel, in other words, isn’t just the power of God to save you, it’s the power of God to grow you once you’re saved. After all, the only antidote to sin is the gospel—and since Christians remain sinners even after they’re converted, the gospel must be the medicine a Christian takes every day."
Tullian Tchividjian, fifteen books on "the gospel for Christians" (read the full thing here )