When Israel Ba'al Shem Tov (Israel the Master of the Good Name) made the first Chasidim, Rabbi Elijah, the Vilna Gaon (the sage of Vilna who was the Gadoll Hador - the elite Torah scholar of his day) banned the movement as a kath or sect. As Judaism is supposed to be non-sectarian that put Chasidism under the same hubris that Christianity is viewed in and as such it falls under the Birchat Haminim or curse of the sectarians a slanderers category.
The Baal Shem Tov told his disciples not to fight the Gaon and he believed that ultimately he would need the Gaon's blessing to continue what he started. Furthermore, it wasn't until the 3rd generation of the movement that Chasidism took off and mushroomed when many Chasidic Rebbes sprang up.
It was the disciple of the Gaon's, Reb Chaim Velozhin, who lifted the ban some time later. The problems that Chabad had some 300 years later must be very well the type of problem that the Gaon was afraid of.
Today, many people who don't know the history of Chasidism seem to think that the Chasidim are the holiest Jews. Basically, they have good public relations.