What is the gospel?
Well, you remember the answer of the Apostle Paul, "It is the power of God unto salvation to every one that believeth" (Rom 1:16).
How easy it is to forget that. How easy to preach it as a system, to preach it as a collection of ideas, or just to preach it as a truth. Ah, but you can do that without power. There are people, says the Apostle Paul, who "have a form of godliness, but deny the power thereof" (2 Tim 3:5).
Christianity is primarily a life. It is a power. It is a manifestation of energy.
--Martyn Lloyd-Jones, Revival (Crossway, 1987), 123