The heretics of the holy light include the Argent Dawn (along with its subsection the Brotherhood of the Light), the Scarlet Crusade, blood elves, fallen priests, and deceivers. These are religious groups that altered the holy light philosophy and splintered from the Church of the Holy Light.
Of the two splinter factions of the Church of the Holy Light — the Argent Dawn and the Scarlet Crusade — the Argent Dawn is by far more palatable to the Church. Still, both technically go against the church's established teachings to blaze new paths.
The Argent Dawn believes the church’s old methods of dealing with evil no longer suffice. Evil constantly changes and refines its methods, and good must likewise change. The Scarlet Crusade takes this notion one huge step forward. The crusade rationalizes that evil has no restraints on its tactics, and neither should good if it expects to triumph. While both groups go against the teachings of the Church of the Holy Light, the church takes no direct action against them. This lack of activity on the church's part derives from its unique structure. While it is the dominant faith among humans, the Holy Light is not a religion so much as it is a philosophy. The "church" is a network of the faithful that exists to bring worshipers together and provide new members with guidance. Adherents to the path of the Holy Light don't attend services once a week or build edifices to celebrate their philosophy (with a few exceptions). Plus, the Church of the Holy Light — and its adherents — have larger problems to worry about.
In addition, while their doctrine might be radical, none can deny that the Argent Dawn and the Scarlet Crusade accomplish what they set out to do. The Argent Dawn has become the preeminent enemy of evil in some areas, and the Scarlet Crusade deals blow after vicious blow to the Scourge. While their methods may be questionable (particularly in the case of the Scarlet Crusade), their accomplishments dissuade most faithful of the Holy Light from interfering.
Blood elves have a heritage with the Church of Light. Many were members of the church when they were high elves. Many of these went on to become light wielding priests, paladins and blood knights. They have also altered their philosophy from that of the Church. At first they forcibly took their power of light from a naaru.