Let’s put this in simple words.
What differentiate practicing asanas (set of physical exercise) in yoga from gymnastics?
Connecting your breath to the movement or the static retention of the poses.
Where does the word spiritual come from?
It’s origin is in the Latin word Spiritus, which has 5 meanings:
As the first meaning of spiritual is simply breathing, per definition yoga – as well as life – is spiritual.
Wasn’t that easy? 😉