Most adults do not believe in Santa Claus, to them he is a myth that is meant for children only. Children place their whole trust in the myth that Santa will bring them whatever they wish. Once it is discovered that he sometimes fails to fulfill expectations they stop believing.
Today I read in scripture some passages where God asks why His people stopped believing in Him. Sounds a bit familiar, doesn't it? There are plenty adults who believed in God as children, but because they were not granted all they wanted from Him they turned their hearts away from the belief that He exists and is good.
Disappointments are the pins that pop the balloons of belief. A belief in Santa or God that disappoints will deflate our hopes and cause us to choose a path that seems safe but will lead to loss, and a hardened heart. Why do we think we should have whatever we want? Why do we think that a wise, good and loving God should answer every prayer on our terms? Is it possible to believe in a good, loving and wise God even if He never answers a prayer in our favor? I think we must say "Yes!"
God exists without our needing Him to exist. He is because He is God! Our unbelief does not cause Him to change or become a myth like Santa. We can believe in God!