"One should love even a Jew like this"

...the Sages state that the churban (destruction of the Holy Temple) was due to the sin of sinas chinam (unwarranted hatred); this means lacking love for your fellow for no reason, with no basis. We, all Jews, want to come out of golus as soon as possible, and so therefore it behooves us to do away with the reason that caused the golus, the tikkun for which is "ahavas chinam" (unwarranted love), which means to love a Jew that the Jewish nation does not benefit from at all, and it is unknown whether any Jew will ever benefit from him, and there is a doubt whether it is possible to apply to him the verse "you shall love your fellow as yourself" because he is not your fellow, r"l, for he doesn't learn Torah and doesn't fulfill mitzvos; but "ahavas chinam" comes [and instructs] that one should even love a Jew like this. And when we behave in this way, we do away with the reason that cause the churban and golus, and through this a Jew illuminates his corner, and automatically light is generated in his portion of the world and additional light, until in the end he will illuminate the golus and we will merit the true and complete geulah in our time, via Moshiach Tzidkeinu.

Letter of the Rebbe, 4 Menachem Av 5711 (Igros Kodesh 4:405, letter #1124)