Nachamu, Nachuma - Perek 40
Perek 40 is the famous Haftorah for parshas Vaeschanan and the first of the 7 prophecies of comfort. This is one of the shifts in tone of the sefer as we move away from rebuke into the prophecies of comfort.
Through Fire and Water - Perek 43
In this perek Yeshayahu makes a powerful statement.
מג,ב כִּי-תַעֲבֹר בַּמַּיִם אִתְּךָ-אָנִי, וּבַנְּהָרוֹת לֹא יִשְׁטְפוּךָ: כִּי-תֵלֵךְ בְּמוֹ-אֵשׁ לֹא תִכָּוֶה, וְלֶהָבָה לֹא תִבְעַר-בָּךְ
2 When you pass through the waters, I will be with you, and through the rivers, they shall not overflow you; when you walk through the fire, you shalt not be burned, neither shall the flame kindle upon you.
sWhat is Yeshayahu referring to?
The Abarbanel says something truly shocking. He explains that Hashem promises to be with us and help us even through the fires of Auto de Fe and the waters of forced baptisms. The thousands of Jews that the Abarbanel witnessed forced into conversion in Catholic Spain and the hundreds burned or killed, were not alone. Hashem was with them.
From the Halachik standpoint, this pasuk is actually the unusual basis for the ritual practice of Tachanun on Monday and Thursdays in the shul. Here is the history.
Sefer Taamei Haminhagim, 136 (quoting the Kolbo)
This story is a metaphor for Jews in exile and acknowledging that Hashem is always with us. It also gives a greater relevance to why we say a longer Tachanun on Monday and Thursday. It is not something to be avoided but rather something to be greatly appreciative for.