Gaudi was intensely religious.  He devoted much of his energy and life to Sagrada Familia, as well as to work on existing cathedrals in Barcelona and Palma, Mallorca.  There must have been some tension between his exuberant architechtural vision and the staunchly conservative Catholic Church in Spain, but perhaps the inherent independence of Catalan thought made it possible to proceed with the Sagrada Familia.  It is certainly not a staunchly conservative structure except in the sense that Gaudi controlled his exuberance sufficiently to create a structurally sound building.  

Gothic cathedrals implied, by their great height, that the Church offered a means to ascend to heaven, at least spiritually.  Sagrada Familia has the height to imply the same thing.  And whether it was intended or not, the tapering towers suggest additional mystery as their top spires recede from view to someone standing at their base.  

To modern eyes, the spires resemble Art Nouveau spaceships for a 1950s space movie with an unlimited production budget.  Georges Melies might have designed them to voyage to the moon (A Trip to the Moon), Jules Verne might have dreamt them decades before, and Sherlock Holmes might have used them to confront Moriarity in some ultimate conflict to save civilization.   If Alphonse Mucha or Aubrey Beardsley had been religious architects, they might have drawn the plans for Sagrada Familia. 

Gaudi was not the original architect for Sagrada Familia, but he took over in 1883 and redesigned it completely.  It was intended to have 18 spires.  It may be completed in 2026.

Mucha's stained glass

Beardsley's vision.

Georges Mieles's Trip to the Moon.

Richard Branson's Virgin Galactic spaceship.