The tefillin are a beautiful mitzvah (actually two, as the hand tefillin and head tefillin constitute two separate mitzvot.) Surprisingly, the Talmud puts what would seem to be disproportional emphasis on this precept. "The skull that has not borne the tefillin, has no share in the World to Come", and "One who does not wear tefillin is as one who sins with his body". Why the dire statements?
Let's look deeper. We take an animal, with all the behaviors that are normal to an animal. The animal is slaughtered for food.The skin is removed. It is washed, soaked in chemicals that strengthen and preserve it (tanning). It is then purified by scraping to remove remnants of blood vessels. The skin, now parchment, is given a silken texture, ruled to receive the writing. The holy passages of the Torah are written on this parchment, including the Great and Holy Name, twenty one times in both the head and hand tefillin. The compartments (battim) are likewise from an animal. The purification of the skins, and the cubic shape. reminiscent of the altar in the Temple, take, in good quality tefillin, over a year to achieve. The passages are placed in the battim, wrapped in the hairs of a calf's tail, and sewn with sinews. The straps are leather as well. Think of what we have just done. We have taken an animal, and transformed it into a great and holy mitzvah. When placed on the arm and head, they become a means of attachment to G-d. From animal to G-d!!!!
Science tells us that we, too, are animals. Torah teaches us that Man is in G-d's image. The Talmud enumerates different ways in which Man is like an animal, others in which he is like an angel. The Tefillin are the ultimate symbol of the transformation from the animal to the Divine. Our bodies DO NOT belong to us to do with them as we please. They belong to G-d. We are meant to soar higher than the eagles; higher than the angels. The tefillin teach us that this transformation is not only possible, but essential. "Sins with his body"? Yes, if we do not recognize the body as a vehicle to our higher selves, we sin against it, ourselves and G-d. "No share in the world to come"? Yes. In Judaism, one does not step out of the body to find G-d. We purify and sanctify the body, thereby finding G-d, as well as ourselves.