I believe that there is actually no set Christian term (as of yet) for the phrase "god of relativism" as bruised reed seems to suggest in his comment. The subject of relativism is slowly becoming a more popular item of discussion in many Christian circles. I am sure that the term god of relativism fits in quite well in Tim Keller's book.
I have heard the phrase “Tyranny of Relativism” as an expression for relativism.
Here is the most common expression that I am aware of involving the subject of relativism. Cardinal Ratzinger (Pope Emeritus Benedict XVI) seems to have made the following expression about relativism rather popular in Catholic circles at least: dictatorship of relativism.
In his homily at the Mass preceding the conclave that quickly elected him Pope Benedict XVI, then-Cardinal Ratzinger said that "relativism, which is letting oneself be tossed and ‘swept along by every wind of teaching,’ looks like the only attitude [acceptable] to today’s standards." He warned: "We are moving toward a dictatorship of relativism which does not recognize anything as for certain and which has as its highest goal one’s own ego and one’s own desires."
Relativism comes in a variety of shapes and sizes. For example, cultural relativism holds that "truth" is merely the creation of a particular culture and what is "true" for for one culture is not necessarily "true" for other cultures. There is moral relativism, the belief that morality is a subjective social creation of a particular people in a certain time and place–and that morality can be changed as desired or needed.
Situational relativism asserts that what is "right" and "wrong" depends on the specifics of each situation–not upon objective, transcendent morality. And cognitive relativism is the philosophical belief that truth, rationality, and knowledge are relative–there is not such thing as objective, definitive truth. - Relativism 101: A Brief, Objective Guide
A quick google search of the phrase Dictatorship of Relativism will procure the following results.