"While some people do have to struggle hard before ceasing to be (say) a Christian, an awful lot simply drift away without much reflection. "

While this ostensibly seems true, these people without reflection carry with them the same baggage from where they drift away.

Belief in something doesn't automatically suggest a passion but could be an unquestioning position not well thought out.

The problem with "mystery" and "powers unknown" is that they are exploited by manipulating religious types to convince others that what they believe is valid.

The "mystery" is always a quest for mankind to discover and make visible what is mysterious. It's like the monster in the closet as seen by a child, the scientist opens the closet door an reveals that mystery as no monster but a scientific fact. Curiosity is inherent in our DNA and solving mysteries is a major preoccupation of the brain.

Myths can be useful as long as that is how they are acknowledged, not truth nor fact but as stories to shed light on human behavior. Folklore serves this function.
Folklore is not history.

The real exultation of life is knowing more about the universe through scientific knowledge resulting in the appreciation for where and how we live. The wonder of little droplets creating the rainbow from the hydrological cycle, the experience of looking at the stars as an index into the past, the miracle of how we got to be here through the gene pool, the astonishment at the revelations of the insect world, the knowledge of aerodynamics as we watch a flock of geese, the poetry of science which is far more substantial then the limited world of "mysterious imagination".