The basis for much of this deceit is that what we hear is comforting. This is often quite apparent in the realm of advertising. To illustrate, many believe in reincarnation and many in bodily resurrection. These are in opposition because they propose two different answers to a physical reality, death. It is true that truth is relative to the individual, but truth is not relative to physical realities.
Physical realities have only one absolute truth.
A visible illustration is gravity. In my world I can believe that gravity does not exist, but if I were to test this by jumping out of a window, I might soon become a convert realized that gravity is indeed an absolute reality on this earth. Indeed, different people find different answers to life and death, but since these concern physical realities, some, if not all of us have to be wrong because of contradicting beliefs. The reason why I am so convinced that the God of the Bible is an absolute reality is because I've tested His words over and over again and have found no fault or contradiction. The Bible makes thousands of claims that are testable.