Ultimately, we realize that we are the same as the universe, which is the Dharmakaya, the Buddha Consciousness that comprises all of existence. This Buddha is also the original Buddha—the Adi Buddha.
Thus in Tantric practice it is important to recognize the connection, or equivalence, between the microcosm (our one body) and the macrocosm (the entire divine universe). Understanding that they are the same thing is considered “mystical” because it involves a union between ourselves and the universe.
So, in Vajrayana ritual, we use our own bodies (through actions with mudra, mantra, mandala) to achieve a union with the divine reality that is the cosmos around us, thereby realizing our Buddha-nature.