For some religious theists, their religion and religious beliefs are so important to them that they can't imagine anyone being without a religion of some sort. For such people, religion occupies such a fundamental and necessary aspect of how they live that they insist the same must be true of everyone. Thus everyone must have a religion of some sort and it's impossible for anyone to live out one. This includes irreligious atheists who insist that they really don't have any religion.
Read Article: Myth: Religion is Central to Human Existence, Everyone Is Religious