I have been practicing yoga for over 30 years and during that time I have seen many changes in attitude and understanding around it shift. When I began my practice many questioned whether yoga was a religion. As individual experience of yoga grew, people realized that it is a spiritual practice that has nothing to do with organized religion but could enhance individual spirituality no matter what your core beliefs.
Another shift that has taken place is that in the past it was easier to have a group of people commit to their yoga practice over time. As a teacher this made it easier to introduce theory and concepts and deepen knowledge and understanding. One session built upon the other. The energy of the class became a container for spiritual growth. If a new person joined the group the review process would further deepen the spiritual understanding of those who had experienced it already.
What seems to be more of an issue teaching at the moment is that many people view Yoga as an element of an exercise program and not a spiritual practice. This may be due to the fact that many gyms are teaching yoga and promoting it as such. Yoga is good exercise but when it is taught in it's totality it is exercise not only for the body, but for the mind, and development of the spirit as well. I am surprised when students do not know the definition of yoga. When I teach, I start my classes with students breathing in the energy of yoga. The energy of unity, The energy of connecting oneself to the divine source within. I teach that this can be a very personal definition. I hold space for the spirituality of yoga to flow.