I think the concept of Reiki being Satanic can be rejected. While i can not speak specifically on Christianities position on Reiki. I did come across this article from a Jewish perspective, maybe it can give some perspective...
Moshe Busch practices and teaches Reiki with a free conscience. In fact, Busch was introduced to Reiki when his hasidic rebbe sent him to a practitioner for healing.
“Judaism and Reiki are totally compatible. Reiki came from the same source [as Judaism], from God,” Busch said.
Busch adds a Jewish element to Reiki by pairing each Japanese symbol with a Jewish symbol, like the magen david, to further strengthen healing.
Busch conducted extensive research into the origins of the Japanese symbols used in Reiki and says he found nothing that contradicts Judaism. He explained that the Japanese characters are simply letters that are not innately charged with energy – they acquired the power to heal only as a result of Dr. Usui’s meditation.
Busch’s practice of adding Jewish symbols to a traditionally Japanese system and practitioners who promote another healing method called Kabbalistic Reiki raise eyebrows among traditional Reiki practitioners who discourage melding belief systems.
Busch chooses to use the Jewish symbols concurrently with the Japanese symbols because he says he gets better healing results with the combination.
“When you are going to help someone, it’s better if you have a lot of tools. For me, using the Jewish symbols is another powerful tool to help someone,” Busch said.
