Tisha B'Av, which is coming up in less than a week, commemorates a plethora of cataclysmic events that have befallen the Jewish people on the 9th of Av. The two most important ones are the destructions of the First and Second Temples. The Bablyonian Talmud, and more specifically Yoma 9b, goes into why each Temple was destroyed:
The First Temple was destroyed because murder, idolatry, and giluy arayot (commonly translated as licentiousness or sexual immorality) were rampant. The Second Temple, however, was brought down by שנאת חנם, or baseless hatred.
First, I would like to propose whether the hatred of individuals can provoke the destruction of a Temple. It can, and it did, especially when that hatred permeated to the highest levels of society and touched the institutional levels. Right before the destruction of the Second Temple, there was such fragmentation amongst the Jewish people. Sectarian quarrelling could not even allow one Jew listen and understand their fellow Jew. Yoma 9b states that the Second Temple should not have fallen because Jews were occupying themselves with Torah, observing the mitzvot, and practicing charity." However, the hatred was so strong and so prevalent that both from theological and historical perspectives, they brought the demise of the Second Temple.
I would like to point something out. First of all, the destruction of the First Temple took three sins. And these aren't any three sins: these are the three for which, traditionally speaking, one would martyr himself rather than commit it (Sanhedrin 74a). The Second Temple, on the other hand, took just one sin: שנאת חנם. Again, the Jews of this time were immersed in Torah and mitzvot (Yoma 9a), but that didn't save them from the ugly fate that came with שנאת חנם. The fact that שנאת חנם is just as powerful as murder, idolatry, and sexual immorality combined shows the immense power of שנאת חנם.
Let's take that a step even further. Not only do we feel the intensity in the fact that שנאת חנם is as powerful as three grave sins. The length of the exile caused by each destruction is even more telling. After the destruction of the First Temple, the Jews were without a Temple for 71 years. But how about after the destruction of the Second Temple? Wait a minute......I don't ever recall there being a Third Temple in our time. As a matter of fact, we have been without a Temple for 1,960 years, and we are still counting. As of date, the second exile has been going on nearly 28 times longer than the first!
Whether you take this Talmudic passage at face value or not, one thing should be clear--שנאת חנם can have tremendous effects on the world. We have been in exile 28 times longer than our ancestors who experienced the first exile, and we still can't get it right. This level of needless, baseless animosity is not healthy for the individual, and it's not healthy for the Jewish people. We can take this lesson in many ways. Love your neighbor (Leviticus 19:18). You shall not hate your brother in your heart (ibid 19:17).
On a day of national mourning, one that is supposed to bring us in touch of national consciousness, we need to realize that this lesson applies to all Jews. When Jews, as a collective whole, can strip away the שנאת חנם, whether it would be with family quarrels, inter-denominational bickering, or even how some Jews feel about non-Jews, when we strip away baseless hatred from ourselves, we can become the "light unto nations," and when we do that, we can hasten the construction of the Third Temple and help bring about the Messianic redemption that we Jews have been desiring for so long.