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1) "To fear the L-rd your G-d all your days"- This refers to Shabbat and festivals (Yevamoth 93).
2) "Fires are common only where there is desecration of the Shabbat" (Shabbat 119b).
2 THE HOLINESS OF SHABBAT
3) "Jerusalem was destroyed only because the Shabbat was desecrated" therein (ibid.).
4) When Rabbi Shimon ben Yochai would see his mother talking too much on Shabbat, he would say to her, "Mother, today is Shabbat (Yerushalmi Shabbat 15:3)."
IDOLATRY AND DESECRATION OF SHABBAT HAVE THE SAME GRAVITY
5) "We may accept sacrifices from Jewish sinners so that they will repent, except from an apostate who offers wine as a libation to idols, or a flagrant violator of the Shabbat. From here we deduce that idolatry and desecration of the Shabbat are of equal gravity" (Eruvin 69b).
6) "I would think that honoring one's father and mother would supersede the Shabbat. Scripture states, therefore: Each man shall fear his mother and father, and you shall observe my Shabbatot" (Yevamoth 5b).
7) "One who casts excessive fear upon the members of his household, will eventually desecrate the Shabbat" (Gittin 6b).
"Said Haman, "if you wish to uproot them, uproot the Shabbat and curtail it, afterwards you will destroy them" (Esther Rabbah 7).
9) "Tur Shimon was destroyed because they would play ball on Shabbat" (Eichah Rabbah 2:3).
10) "Desecration of Shabbat is one of three sins for which their perpetrators are blamed for committing all sins." (Midrash Tannaim, Deut. 5).
11) "Whoever desecrates Shabbat, although he possesses Torah and good deeds, has no share in the World to Come" (Avoth d'Rabbi Nathan, ch. 26).
IT IS CONSIDERED AS VIOLATING ALL OF THE TORAH
12) "If you desecrated the Shabbat, I (G-d) consider it as though you desecrated all the commandments" (Shemoth Rabbah 25)
13) "Whoever desecrates the Shabbat, testifies before He Who spoke and the world came into existence, that He did not create His world in six days and rest on the seventh" (Mechilta Yitro)
14) "Any Jew who did not observe Shabbat in this world, has no rest to eternity. One official takes his body and brings it into Gehinnom in the presence of the wicked, where his soul has no rest in the fires of Hell. When Shabbat is over, that official returns his body to its place, and each is tortured individually" (Midrash quoted by Maavar Yabbok, Sifthei Renanoth 39).