If the gospel is merely assumed, while relatively peripheral issues ignite our passion, we will train a new generation to downplay the gospel and focus zeal on the periphery. It is easy to sound prophetic from the margins; what is urgently needed is to be prophetic from the center.
--D. A. Carson, 'The Gospel of Jesus Christ (1 Cor 15:1-19),' p. 2
This was an orally delivered lecture given at the Gospel Coalition in May 2007 and is the best essay-length summary of what the gospel is I know of. See also Dr. Carson's 'The Biblical Gospel' from the 1996 For Such a Time as This.