The history of the Sith starts almost twenty-five millennia before the events of Star Wars: Episode I. The Jedi Order formed during this time along with the creation of the Galactic Republic. Up until that point, the Force was practiced in varied and undisciplined ways. On many worlds the Force was misunderstood and often considered to be magic. Force adepts were either revered and worshipped, or shunned and feared.
The founding members of the Jedi Order were Force-sensitive individuals who understood the power and potential of the Force and also foresaw its abuse and danger. They agreed to pool their knowledge of the Force, continue to study it, and most importantly educate and regulate those who possessed the power. The Jedi Order was dedicated to making sure the Force was used to ensure peace in the galaxy.
Though the dark side of the Force was not well understood by the Jedi, they were aware of its presence. Early Jedi had all felt the lure of the dark side and agreed that the dark path was not one ever to be tread upon by a Jedi, not even for a moment.
However, not all of the early Jedi agreed.