A human being is part of the whole, called by us "Universe", a part limited in time and space. He experiences himself, his thoughts and feelings as something separate from the rest - a kind of optical delusion of his consciousness. The striving to free oneself from this delusion is the one issue of true religion. Not to nourish it, but to try to overcome it is the way to reach the attainable measure of peace of mind.
~ letter 12 February 1950 from
Albert Einstein to Robert S. Marcus on the death of Marcus's son from polio.
I found this remarkably clear and simple