Postmodernists reject any notion of grand "truth", such as say belief in God or the narratives associated with God. Similarly, the idea or narrative that science is going to improve our lives is rejected also. Indeed any grand narratives are held suspect. For postmodernists "truth", if it exists at all, is all relative and may only be found in little stories or micro narratives and experiences of ordinary people and those living on society's fringe. No one religion has the answer as each faith group has its own beliefs - and "may" have some "truth" on its side. The main French theorist who advances this idea is Jean-Francois Lyotard (1924-1998).
Of course, you do not need to believe in any of this, as this comedian makes clear. And in any case, to argue that there can be no "truth" is somewhat contradictory as one is trying to advance a grand "truth" that there is no "truth"!