Because [St.] Luke was trained in the medical arts of his day, he undoubtedly recognized the important spiritual component to physical health, as did traditional healers in most ancient societies. It is fitting, therefore, that the bull or ox is the animal associated with Luke, because it is recognized worldwide as having great physical strength and, in many religious traditions, as an animal of great spiritual power and mystery.
As a devotional practice, find a small representation of a bull and put it in a place of honor to witness to the union of body and spirit, the physical and the spiritual. Let it remind you that total health is a product of the spirit and the body working in harmony with each other and with the Divine Spirit in nature. As an animal of power, strength, and mystery, look upon the bull as a representation of the Divine Spirit, which exists in all things and which can manifest in your life in the form of a strong, powerful bull as well as a gentle dove.
— Tom Cowan in The Way of the Saints