The main mitzvah upon which all the other mitzvot are structured is emuna, certainty of G-d. The Gemara tells us (Makkot 24) that the Prophet Habakuk "summed up" all the mitzvot into one, saying, "The righteous will live by his faith." Therefore, since we are obligated to teach our children to fulfill the Torah's commandments, it is clear that we must teach them emuna from a very young age – for that is the mitzvah on which everything else stands.
The three weeks that begin on the night of the Seventeenth of Tammuz and continue until the Ninth of Av are days of national woefulness. The sages therefore advise being especially careful during this period, for it is a time designated for calamity.
Just as “cleanliness” must reside in deeds, so must it reside in traits. The work involved in changing one's traits is more difficult than that involved in changing one's deeds. This is because changing traits means doing battle with one's own nature.