Q: Why is failing to wear Tefillin such a severe transgression that the Gemara (Rosh Hashanah 17a) says: "Who are the sinners of Israel? Rav said: a skull that did not wear Tefillin"? Can someone who did not put on Tefillin rectify their sin?
A: Some explain that "a skull that did not wear Tefillin" refers to someone who did not wear Tefillin even one time during his lifetime (Rabbenu Chananel ibid. and Rif ibid. 4a). Others explain that the reason he never put on Tefillin is because the Mitzvah is embarrassing to him. He does not want to put straps and boxes around his head and on his arms (Tosafot ibid. and Rosh ibid. 1:5). And there are still others who explain that this statement does not only apply to Tefillin but to any mitzvah that is done with one's body, since it is forbidden to shame any mitzvah and it is a disgrace not to fulfill it even one time (Ron ibid. in pages of the Rif and Korban Netanel ibid. #10). The Tosafot there, however, did mention that if someone did not put on Tefillin, he is able to repent. Everything can be repaired. Nothing stands in the way of repentance even in the case of severe transgressions. The Tosafot said that if he repents, the repentance will hang in the balance until he dies and the suffering of death will complete the process of repentance. There are people who stand in the street and try to get Jews to put on Tefillin even if they will not do so again. The last Lubavitcher Rebbe – Ha-Rav Menachem Mendel Schneerson – explained that a person should put on Tefillin at least once in his lifetime so he does not have "a skull that did not wear Tefillin" according to the first explanation (see Sha'arei Halachah U-Minhag vol. 1, p. 84).