I think the bumper sticker is talking about the philosophy of non-violence that Jesus, Ghandi, and MLK all promoted in different ways, and which is seemingly in line with a more liberal foreign policy perspective. Liberals are generally more critical of a foreign policy based solely on military might and violence, and lean more toward a posture of international collaboration. Some write off people with such a collaborative world view as “just a bunch of liberals,” as though that means they are nuts and out of touch with reality. The bumber sticker is thus pointing out the hypocrisy of Christians and others who hold up MLK and Ghandi, but then write off current folks who promote a more non-violent approach as a bunch of nutty liberals.
The interesting thing about this bumper sticker is that it inadvertently reaffirms the notion that “liberal” is now a bad or negative word to the general public, while obviously being a bumper sticker a person of liberal persuasions would have on their car.