Adding something cant subtract from the totality of a person, it has to make the person something more and closer to their full potential. It is like teaching a person to speak, or read, or think logically, or speak a second language, or understand maths or poetry, or how to play a musical instrument.
So, no matter what a person is without a spiritual dimension they are more when they acknowledge and embrace one. They can see things in alternative ways and think using a different model of thought. They are linked to humanity and the universe around them via the divine spirit however they perceive that to be. They are given greater empowerments, physical and psychological, either via the power of belief, or via the abilty to tap into the physical powers of god/the divine/the cosmic consciousness.
One can only be more, and closer to full human potential, when one develops a spiritual awareness or a connection to the divine.
Tha t does not diminish the many wonderful people like my father or yours who had no such connection or perception. My father was a secular humanist with the emphasis on active humanism. He acted how truly spiritual and religious people are supposed to act, yet without a recognition of those forces motivated, rather, by the human spirit and a value/philosophical based system of ethics and moralities.