By definition, a “spirit” is the immaterial thing that animates a body. If your body is alive, then you have a spirit.
Another more profound definition calls it “an attitude or principle that inspires, animates or pervades thought, feeling, or action.” Notice how all-encompassing that is: the spirit is the part of our being that infects every inch of your thoughts, feelings, and actions as it animates them, bringing them to fruition. Is there a single attitude or principle so potent that it sustains your entire thought life as it fuels your every emotion and drives you to carry out all your actions?
To be united with Christ on a spiritual level would mean that our thoughts, feelings and actions would all be animated by the same attitude/principle that animated those of Christ. This union of spirit would pretty much make us a replica of Christ as his spirit lives in us, animating our beings in the same way that it animates his.
