An excerpt from Tim Keller from the 2006 Desiring God National Conference:
We never “get beyond the gospel” in our Christian life to something more “advanced.” The gospel is not the first “step” in a “stairway” of truths, rather, it is more like the “hub” in a “wheel” of truth. The gospel is not just the A-B-C’s of Christianity, but it is the A to Z of Christianity. The gospel is not just the minimum required doctrine necessary to enter the kingdom, but the way we make all progress in the kingdom.
[...] How then when thinking about preaching form can one reach out without selling out? This is not to mean only reaching out to those you are not Christians but to all those who gather. Christian and non-Christian alike need to hear the gospel. “We never go beyond the gospel.” [...]