One of the special haftorahs of the "Three Weeks" is an excerpt from the Book of Jeremiah in which the prophet relates how G-d instructed him to foretell of the destruction of the Holy Temple.
Jeremiah lived in a time when many Jews were attracted to paganism; his function as a prophet was to arouse them to repentance. Fearful of undertaking such a responsibility, G-d encouraged Jeremiah with the following words: "Before I formed you in the belly I knew you; and before you came out of the womb I sanctified you, and I ordained you a prophet to the nations." When Jeremiah countered that he was only a "child," G-d replied, "Do not be afraid of them, for I am with you to save you."
In essence, Jeremiah's mission is the mission of every Jewish soul, which is forced to abandon its G-dly Source and descend into the physical world. The soul becomes frightened at the prospect; how can it possibly contend with all the difficulties it will encounter?
G-d immediately reassures the soul and tells her not to be afraid: "Before I formed you in the belly I knew you." Every Jew has a Divine soul, "a veritable part of G-d Above" that transcends the physical world and the difficulties of the exile. Moreover, "before you came out of the womb I sanctified you": every Jew is prepared ahead of time by having been taught the entire Torah before he was born.
This, however, is not enough to assuage the soul's fears. "But I am only a child!" it counters. "From where will I get the strength to be a prophet to the nations?" i.e., to refine and elevate the physical plane of reality?
"Do not be afraid of them, for I am with you to save you," G-d promises. In addition to the innate powers you acquired in the womb, I will give you special strengths and abilities to be able to fulfill your mission successfully.
