The centrality of the cross
From Tim Keller
We are not justified by the gospel and then sanctified by obedience, but the gospel is the way we grow (Gal.3:1-3) and are renewed (Col.1:6). It is the solution to each problem, the key to each closed door, the power through every barrier (Rom.1:16-17). It is very common in the church to think as follows. "The gospel is for non-Christians. One needs it to be saved. But once saved, you grow through hard work and obedience." But Col.1:6 shows that this is a mistake. Both confession and "hard work" that is not arising from and "in line" with the gospel will not sanctify you--it will strangle you. All our problems come from a failure to apply the gospel. Thus when Paul left the Ephesians he committed them "to the word of his grace, which can build you up" (Acts 20:32)