Yin/Yang - Is it a religious symbol?

Short answer:

No.

Long answer:

It is true that in the Orient the principle of yin and yang has been applied in many endeavors, including divination. However, this would not rule out its use in other ways that are not prohibited in the Scriptures, such as in making a medical diagnosis.*

As is true with most arts of Southeast Asia, the yin-yang doctrine has a pervasive influence in the theory and practice of Chinese medicine. In some instances, yin simply represents cold and yang represents hot—but also, they can represent many other opposing properties.** An example of this in Traditional Chinese Medicine (TCM) would be herbs and foods that are considered either cold or hot and can be prescribed by an herbalist to counteract the yin-yang imbalance in the patient.

In Chinese philosophy, yin and yang describe how seemingly opposite or contrary forces may actually be complementary, interconnected, and interdependent in the natural world, and how they may give rise to each other as they interrelate to one another.