Over at Midwest Christian Outreach Ron Henzel, Christian apologist and Senior Researcher at MCO, has an edifying post concerning the cardinal Christian doctrine of the vicarious penal substitutionary atonement of Christ on the Cross: 

Christian worship services around the world regularly echo with the declaration of John the Baptist: “Behold, the Lamb of God who takes away the sin of the world!” (John 1:29, NASB). In some services this verse is quoted as a regular part of the Lord’s Supper, or communion service. At other times it may be part of an evangelist’s call to unbelievers in his audience to trust Christ for salvation.

One can only speculate as to the immediate reaction of those who first heard it from the lips of John as he pointed out the peasant frame of Jesus of Nazareth to them. Where they not preparing for their king? Why should they stop to consider a lamb?

In the grand sweep of biblical narrative, it is a declaration both simple and profound—simultaneously concealing and revealing in language that is both cryptic and commonplace. Did anyone who heard it that day try to consider its ramifications? If the Messiah was to be a lamb, that meant he was to be a sacrifice—specifically, a sacrificial victim. Anyone who lived anywhere near an altar, either Jewish or pagan, would have understood that. (Online source)