The Shulhan Aruch (Orah Haim 37:1), in discussing the laws of Tefillin, emphasizes the special importance of this Misva, writing that those who wear Tefillin are promised great reward (listen to audio recording for precise citation). A number of sources teach that proper observance of this Misva earns a person long life, as well as good health. The Tefillin Shel Rosh is situated near the brain, and the Tefillin Shel Yad is worn near the heart – the body’s two life-sustaining organs. Hence, if one properly observes this Misva, he is rewarded with life and health.
The Gemara mentions numerous rewards that a person earns by ensuring to wear Tefillin and Sisit and recite the Shema and daily prayers. Such a person is guaranteed a share in the next world, spared from the punishments of Gehinam, and achieves atonement for his sins. Other sources note that the Tefillin Shel Rosh, which is situated on the head, atones for arrogance, and the Tefillin Shel Yad, worn on the arm, atones for even grievous sins like murder. It also atones for the sin of "Shichbat Zera Le’batala" (wasteful semenal emissions). The Zohar comments that the importance of this Misva equals that of all other Misvot combined.
Besides emphasizing the great rewards earned for observing this Misva, our sources also warn of the punishments which could befall those who neglect this Misva, Heaven forbid. The Shulhan Aruch writes, based on the Talmud, that one who does not wear Tefillin is classified under the category of "Posh’eh Yisrael Be’gufan" – "Jews who sin with their bodies" – who are liable to severe punishments in the afterlife. Different views exist in explaining to whom exactly this refers. Some claim that this refers only to those who do not wear Tefillin because they ridicule and look disdainfully upon the Misva and do not afford it any significance. Others, however, maintain that even those who neglect the Misva of Tefillin for other reasons, such as laziness or indifference, fall under the category of "Posh’eh Yisrael Be’gufan." The Kaf Ha’haim (Rabbi Yaakob Haim Sofer, 1870-1939) writes that this applies even to those who eat foods at night that cause indigestion the next day, such that they cannot wear Tefillin (as wearing Tefillin requires bodily cleanliness and refraining from passing air).
Due to the unique importance of this Misva and the gravity of its neglect, the Mordechi (Rav Mordechai Hakohen Ashkenazi, Germany, 13th century) writes that one must ensure to purchase Tefillin from a God-fearing, trustworthy Sofer (scribe). One should also make a point of acquiring the highest quality Tefillin, and not look for "bargains," so that he can fulfill this Misva at the highest possible standard.
Moreover, the Mishna Berura (commentary to the Shulhan Aruch by Rav Yisrael Meir Kagan, 1839-1933) warns that wearing Tefillin improperly is no different than not wearing them at all. One must therefore ensure that his Tefillin Shel Yad is positioned on the muscle of the upper arm, and that the Tefillin Shel Rosh does not drop below the hairline. (If a person had lost hair, he must wear his Tefillin behind the point where the hairline had been.) One who does not wear the Tefillin in the proper position is considered to have not worn them at all, and may thus suffer the fate of "Posh’eh Yisrael Be’gufan."
Finally, the Kaf Ha’haim notes that the blessings and rewards promised to those who wear Tefillin are contingent upon one’s proper conduct while wearing Tefillin. Namely, while one wears Tefillin he must not engage in any kind of idle chatter or frivolous conversation. He must instead keep his mind focused on matters of Torah and prayer and maintain a respectful, serious demeanor the entire time he wears the Tefillin, and he will then be worthy of the many rewards this great Misva offers.
