Faith Ascension differs from mana Ascension in that the Concept being absorbed is artificially created. It differs from qi Ascension in that the cultivator is absorbing an outside Concept to strengthen and alter his own Definition. Faith is a kind of mana, but one that has very qi like properties. It is tied to a specific individual by Definition, and is used by that individual to further his/her Path of Ascension.
Worshippers add to this Faith pool by offering their time in prayer, sacrifices on an altar, or donating money. Each forfeits Potentiality, converting it to the cultivator's Faith pool. Out of all sacrifices, those who are sentient contain the most Possibility, while the young and pure have even more than the old or married just due to the potential choices that are taken away by killing them. This is actually how the evil kind of death magic works, gaining power by sacrificing the Possibilities of other people.
As a side note, blood magic is not as bad as death magic, as it is more about using the qi in the blood for power. Qi is usually in the blood, unless there is a Law otherwise. It can have inimical uses just due to qi being very personal and bound up in a person's Definition, which is where it gets its bad reputation - from voodoo dolls and other curses blood can be used remotely for. Conversely, it can also be used for blessing, so it isn't all bad.
Anyways, there are a couple of reasons people tend to avoid starting cults and using the Faith Path of Ascension. One is because the cultivator's worshippers will Define him, both the good and the bad. Another is because starting cults and cultivating the image you want from them can be very difficult unless you are already powerful by haven taken steps on another Path of Ascension. I should note here that it is very possible for a small minority to do so, it's just difficult.
Finally, an individual cultivator of this Path should ensure that what he preaches to his cult or religion and the impressions he gives his followers is what he wants to become. He might become a goody goody two shoes instead of the womanizer he wanted to be. Even if he influences his cult so that it degenerates into depravity, it would have to be associated with his worship. Otherwise, he might even lose Faith power as his devotees become not devout anymore.