There surely ought to be little difficulty in determining what Christianity is...Unquestionably, Christianity is a redemptive religion, having as its fundamental presupposition the fact of sin, felt both as guilt and as pollution, and offering as its central good, from which all other goods proceed, salvation from sin through an historical expiation wrought by the God-man Jesus Christ. The essence of Christianity has always been to its adherents the sinner's experience of reconciliation with God through the propitiatory sacrifice of Jesus Christ.
Certainly his first followers with single-hearted unanimity proclaimed the great fact of redemption in the blood of Christ as the heart of their gospel: to them Jesus is the propitiation for sin, a sacrificial lamb without blemish, and all their message is summed up in the simple formula "Jesus Christ and him crucified."
No doubt parties have from time to time arisen who have wished to construe Christianity otherwise. But they have always occupied a place on the periphery of the Christian movement, and have never constituted its main stream.
A Christianity without redemption--redemption in the blood of Jesus Christ as a sacrifice for sin--is nothing less than a contradiction in terms.
We may fairly contend that the germ of Christless Christianity is present wherever a proper doctrine of redemption has fallen away or even has only been permitted to pass out of sight. Of course in the meantime some other function than proper redemption may be found for Jesus.
B. B. Warfield, "Christless Christianity," in The Works of Benjamin B. Warfield vol. III: Christology and Criticism, p. 355-8