Let's put aside the fact that Dr. Usui was a Christian preacher or that he discovered Reiki specifically while looking for an answer regarding how Christ healed through the laying-on of hands, which is mentioned numerous times in the Bible and exists in nearly all cultures globally, and instead discuss the chakras themselves.
The chakras, also called Ki Centers (this term will be important later), are located at various physical points on the body. There are minor chakras in addition to the main ones shown in the image but I am not going to discuss them here. Any scholar of theology will understand the importance of numerology. There are 7 main chakras, seven being the number for God Himself as it symbolizes completion and perfection.