From a theological perspective, the message of Christianity is meant to be provocative. Serrano’s image could be seen as a modern sweeping clean of the temple, casting out the modern moneychangers, i.e., those who’ve made a commodity of Christ. Robert Hughes in his American Visions points to Piss Christ as the beginning of the culture wars in America, when the American Religious Right tried to dictate what was and was not art. I see Serrano commenting on the very social climate surrounding his art by placing a cheap plastic crucifix in a container of urine and giving it its vulgar title. In that reading, Christ is cheap and plastic, a fake belief system created by those befouling the truth of Christianity with their vulgar prejudices and power-seeking motivations.
I also see Serrano commenting on the reality of death at the moment he made his image. In 1987, the AIDS epidemic was beginning to take hold of the public consciousness. By placing the most iconic image of death (the crucifix) within the modern context of death (bodily fluids), Serrano links the two, taking the message of hope found in the grotesque image of the crucifix and affixing it to the bodily grossness of urine, here acting as a stand-in for AIDS.
But even more important than either of those readings is the ability of this image to provoke those readings. It shocks you out of your quotidian concept of what an image of Christ should be and makes you see it again. Like all good art (and good theology), it makes you think.