The Cathars do not call themselves Cathars. Insofar as they refer to themselves as a group, they call themselves the believers (credentes).
The faith stems from Christianity. But they believe that the God of the Old Testament, who created the world, is Satan. Earth is Hell. Within every person is a divine soul, trapped in material substance. When people eat, have sex, handle blood and other bodily fluids, they are polluted by the evil of the material world.
Even in marriage, procreation is not desirable. Children are demons. Only when they become conscious enough to choose the right way to live, their souls can be saved. Until then their bodies are just earthly prisons for their souls.
Through an ascetic life of purity, prayer and abstinence people can seek to purify themselves. For most, this is an impossible task. But until it succeeds, the soul is reborn in a new earthly prison, when the body dies. The few, who feel ready for the task, become perfects (parfaits).
Not all believers can live up to the strict demands. The few, who feel ready for the task, become perfects (parfaits). Perfects renounce material possessions except a cowl and a belt. They dedicate themselves to an ascetic life of purity, prayer, preaching and working to help the community.
You become a perfect by going through the purification ritual consolamentum after a long time of preparation and instruction. The ritual can only be performed by perfects. The effect is hereditary: If the perfect, which performed the ritual for you, breaks his vow, the vow is also broken for you and everyone you have purified.
You can only be purified once, so most people go through the ritual shortly before their death, so as not to risk becoming impure again.
As a perfect, you must follow the example set by Jesus Christ and his disciples. Avoid cursing and swearing. To take an oath would be the same as admitting that you do not always speak the truth. Avoid killing. This includes eating meat, since it could be one of your brothers or sisters reborn in the form of an animal.
Help others find the path. Perhaps they can be reborn as perfects in their next life.