The greatness and severity of the Mitzvah:
The reward for the Mitzvah of Tefillin is very great. The following statements are recorded by Admur regarding the greatness and importance of this Mitzvah:
Poshe Yisrael Begufan:
One who does not put on Tefillin [for even one day] is considered within the category of “Posheiy Yisrael Begufan”. [According to Kabalah, such a person is called a Karkafta that does not merit to be resurrected in the period of Techias Hameisim.] Even if he refrains from wearing them due to fear of not keeping a clean body, he has transgressed, as certainly he is easily able to guard his body during the time of Shema and Shemoneh Esrei. [Nevertheless, in such a case, some Poskim rule the person is not called a Posheia if he does not wear the Tefillin due to reasons of cleanliness.]
The Mitzvah of Tefillin is the deciding factor on judgment day:
The Sages taught that on the Day of Judgment, if one is [judged] meritorious in the Mitzvah of Tefillin then the side of merit dominates. However if one was negligent in the Mitzvah then the side of liability dominates.
Is the greatest positive command:
The Mitzvah of Tefillin is the greatest positive Mitzvah in the entire Torah, as the entire Torah is connected to Tefillin. For this reason every person needs to be careful in the Mitzvah of Tefillin.
Merits one long life:
All those which are careful in the Mitzvah of Tefillin will have lengthy days. [This is because the Tefillin correspond to the heart and mind on which one’s life depends. Some say that one is to state Talmudic passage prior to wearing the Tefillin and that doing so is a Segula for long life.]
Sparks of Chassidus:
Partner with Hashem:
One who wears Tefillin is considered a partner of Hashem in the creation of the world.
Elevates one’s prayer:
All of one’s prayers are elevated through one’s Tefillin. When one wears Tefillin during prayer the angels welcome it through announcing “Bring respect to the resemblance of the King.”
Subjugates the heart:
We place the Tefillin by the heart and by the head in order to subjugate the soul that is in the mind and the desires within the heart towards service of Hashem, as through donning Tefillin one remembers Hashem and diminishes his pleasures.
A combating tool for one who suffers from schizophrenia:
Wearing Tefillin helps one subjugate his mind and heart to be in tune with reality. It is hence a great method for battling schizophrenia. The illness of schizophrenia causes a person to have distortion of reality and confuse fact for fiction and fiction for fact. His emotions are linked to the false realities that his mind envisions and cause him to sway from one extreme to another. Wearing Tefillin is helpful in refining the mind and the heart and to recreate a healthy bond between them.
Merits the world to come:
One who wears Tefillin merits a portion in the world to come. The fire of Gihennim does not overtake him and all of his sins are forgiven.
Brings humility and fear of Heaven:
Wearing Tefillin brings one to humility and fear of heaven, and prevents one from thinking foreign thoughts.
Rebbe Eliezer Ben Yaakov stated: Whoever has Tefillin on his head and Tefillin on his arm and Tzitzis on his garment, and a Mezuzah on his door is guaranteed not to sin, as the verse states “Vehachut Hameshulash Lo Bemiheira Yinatek” as well as the verse states “The angel of Hashem rests around those who fear Him, and save him from sin”. [This however only applies to one who internalizes the meaning behind the Tefillin that he is wearing. However one who does not do so, ity is as if he is placing the Tefillin on the wall or table. This is why the Talmud states “Tefillin in the head” and not “Tefillin on the head” as having the Tefillin simply on one’s head does not suffice.]
Places fear onto the nations:
The verse states “And all the nations of the land will see that the name of Hashem is written on you and they will fear you.” The Sages state that this statement refers to the head Tefillin, as the head Tefillin contains two letters of the three letter name of Shakaiy.
Times the Jewish people were lax in this Mitzvah:
The Talmud states that every Mitzvah that the Jewish people were not careful to give up their lives for during times of persecution [Shemad], the Mitzvah is weakly observed until today. An example of such a Mitzvah is Tefillin. We thus find that there were tens of thousands of Jews in Spain and other lands during the times of the Semag [1200’s], and possibly also during the times of the Tur [1300’s], that did not put on Tefillin.
 37/1; Ketzos Hashulchan 8/28
 Admur ibid; Michaber 37/1
 M”B 37/2; P”M 37 A”A 1 in name of Nachals Tzevi 1
Other opinions: Some Poskim rule that one is only considered a Poshei Yisrael if he did not put on Tefillin his entire life. [Rambam Teshuvah 3/5; Shaareiy Teshuvah of Rabbeinu Yona 3/11; Ran Rosh Hashanah 17a]
 Rav Chaim Vital in Shaar Hakedusha 1/7, brought in Kaf Hachaim 37/6
 M”A 37/1; See Machatzis Hashekel ibid; Beir Heiytiv 37/1
Other opinions: Some Poskim rule that even in such a case the person is included within Poshei Yisrael. [Beis Yosef in opinion of Tur; See Kaf Hachaim 37/5]
 Admur ibid; Rosh Tefillin 28
 Admur ibid; Rosh Tefillin 28
 The reason: This is learned from the verse “Lemaan Tehiyeh Toras Hashem Beficha” [ibid; Keddushin 35a]
 Admur ibid; Resih Lakish in Menachos 44a
 This does not just mean that he puts on the Tefillin, but that he is careful to guard all of its laws meticulously amd to purchase Tefillin of Hiddur
 Yafeh Lalev, brought in Kaf Hachaim 37/1
 Mishmeres Shalom Basra 6
 Rokeiach Bo
 Imrei Pinchas
 Tikkunei Zohar Tikun 47
 Sidur Admur; Admur 25/11
 Likkutei Sichos 36 p. 297; “Healthy in Body and in Spirit” Volume 3 p. 138
 Shimusha Raba
 Rambam 4/25
 Menachos 43b
 Brachos 6a
 Beir Mayim Chaim 1, brought in Ketzos Hashulchan 8 footnote 43; Taamei Haminhagim Tefillin 25; Sefas Emes Bo
 Devarim 28
 Rebbe Eliezer Hagadol in Menachos 35b
 See Rashi and Tosafus ibid
 Shabbos 130a
 Shabbos ibid; Semag 3; Rosh Menachos Tefillin 28
 Bach 25 “Veachar”; Ateres Zekenim 25