The Talmud (Yoma 9b) explains the reason for why God allowed the Second Beit Hamikdash, or Holy Temple, to be destroyed: sinat chinam. This phrase is often translated as "baseless hatred," but I have never found that definition to be instructive. After all, anyone can rationalize as to why their hatred is justified. However, there is another way to translate sinat chinam: "hatred of their chein." The reason I find this definition to be the most accurate is because it provides a basis from which any of us can combat this destructive force.
The word "chein" is yet another term that is difficult to accurately translate, but it is generally defined as grace, beauty or charm. In other words, it is referring to the particular quality that makes each one of us unique. Every person, no matter how religious or irreligious, has a unique divine spark, talent or ability. It is our job to make sure we use that gift for good. At the same time, we must allow others to pursue a path of goodness while using their own chein. Herein lies the reason for why sinat chinam is such a serious vice. When one hates a fellow Jew (instead of respectfully disagreeing with them), it's ultimately a lack of recognition of God and the particular grace He has bestowed upon that individual.
In our day and age we seem to be more polarized than ever before. When someone holds a certain view on ritual observance or religious thought, all too often those who hold opposing views begin personal attacks instead of simple disagreement. This is so obviously counterproductive that it should go without saying, but unfortunately it has to be articulated. So here it goes: no matter how strongly any of us disagree with a fellow Jew, the moment we begin unwarranted personal attacks, we have lost the argument and engaged in sinat chinam. The reason for this is because instead of focusing on the substance of the issue (and possibly learning something from the other person's approach), we have attacked their character and unique understanding of life. Both civility and the possibility of bringing them to our way of thinking is lost.
We each have to figure out a way to tolerate those who differ from us in terms of dress, ritual observance, ideology, political affiliation, and other major areas of life. However, there is an equally important point to make: sinat chinam does not include hating someone who violates basic ethical principles (i.e. those which hurt other people). For example, it does not mean tolerating a businessman who engages in a multi-billion dollar Ponzi scheme or a rabbi who assaults women and children. In such cases, we are not hating the person for who they are - a certain type of businessman or rabbi of a particular denomination - but rather for the pain they have inflicted upon innocent people. In fact, it would be a chilul Hashem, a desecration of God's name, to overlook such evils. While tolerance is generally a virtue, it cannot be taken to the point where we drop ethics.
We have been in exile for bordering on 2,000 years as a result of sinat chinam between Jews. This vile and ubiquitous form of hatred has literally destroyed us from within and has spiritually allowed our enemies to attack us from without. There is no greater way to defeat our enemies, and evil for that matter, than to focus on removing sinat chinam and uniting as one people despite all our differences. And it isn't even that difficult. Every individual Jew can go on with their life as usual. We simply have to make sure that when disagreements inevitably arise that we focus on substance, allow the other person their unique approach to the issue, and courteously offer our own perspective. If sinat chinam - hating another person's chein - had the power to destroy God's home on earth, then ahavat chinam - loving (i.e. tolerating) another person's chein - has the power to rebuild it.