The God of the religion would also become less powerful due to the lack of practical worship of him. These religions were true to those who believed in them when they existed but not true after their beliefs or the circumstances in their life became different. See, if religions who claimed inherent absolute truth was at the back bone of their beliefs, were really true, then there would still be the truth when the religion was long gone. Truth is an enduring rock of ages whereas religions will fade in and out as time goes on. The key to this process is tapping into the latent power of the current faith of the masses.
Faith generates energy to put things in motion.
It appears that religion is a true case of majority rules and that the supernatural presences that affect us are the ones that we create. With this understanding, any religion can be created but will not be effective unless it is solidly built enough for people to have true faith in it. For example, there probably won't be anyone casting fireball spells any time soon because there is not a solid belief foundation that people could latch onto to make it happen. Therefore, without the mental visualization and understanding of how it would take place, there would not be any latent faith energy to push such an occurence into reality. I'll stop for now.