Is it necessary to believe in a religion to be happy?
From a Buddhist perspective, to be happy, we have to understand the causes and conditions for unhappiness. If those are clearly understood then we can try to create the causes and conditions for happiness. This is not a matter of belief but rather one of inquiry. This kind of inquiry is not the sole domain of religion but is a part of our common human aspiration. We all want to be happy in whatever way we define that word.
Some of our unhappiness is trivial and some of our unhappiness is tragic. The unhappiness of civil war is different than the unhappiness of losing a hockey game. The solutions we find to deal with unhappiness are also different. Being bored and buying a new gadget gives one a certain kind of happiness. Being bored and volunteering at the local hospice gives another kind of happiness. The results will vary according to causes and conditions.
The Buddha's spiritual journey came about through his own inquiry into suffering, an inquiry into the human predicament of birth, old age, sickness and death. His inquiry was not merely intellectual but deeply spiritual and his realization was equally profound. It is this deep spiritual realization that lies at the root of the serenity and compassion of the Buddha and it is this enlightened realization which he taught.
The great spiritual beings of our human history have all taught that there is a sacred, transcendent value to each human being beyond the circumstances of their birth and culture. It is this that is at the heart of the religious impulse. In Buddhism we call it wisdom. My own teacher put it this way, "The peace which arises from wisdom is not happiness, but is that which sees the truth of both happiness and unhappiness".