Pulling from Machen's "Christianity and Liberalism," Carrick sifts out three wonderful, pithy quotes -
"Christianity begins with a triumphant indicative"
"Christianity is a religion founded not on aspirations, but on facts..."
"The liberal preacher offers us exhortation...The Christian evangelist...offers...not exhortation but a gospel." - from Carrick, IP, p7
The gospel is not first of all something to do; it is first of all something which has been done.