But if we walk in the light, as he is in the light, we have fellowship one with another, and the blood of Jesus Christ his Son cleanseth us from all sin. 1 John 1:7
"Blood’s quality of cleansing appears throughout the Bible, from the earliest books to the latest. In Leviticus 14, for example, a priest sprinkles cleansing blood on a person with an infectious skin disease and on the mildewed walls of a house. New Testament authors often refer to Jesus’ blood “cleansing” us (e.g. 1 John 1:7), and Revelation describes a multitude who “have washed their robes and made them white in the blood of the Lamb” (Rev. 7:14).
Does this frequent reference to blood indicate primitive Christianity’s remoteness from modern culture? To the contrary, modern medical science has shown that the symbol of cleansing conforms closely to the function of the actual substance. Presumably biblical writers did not know the physiology behind their metaphor, but the Creator chose a theological symbol with an exact analogue in the medical world. All that we have learned about physiology in recent years confirms the accuracy of the still-jarring linking of blood and cleansing. The theological image makes for good biology as well.
If you truly wish to grasp the function of blood as a cleansing agent, I suggest a simple experiment. Find a blood pressure test kit and wrap the cuff around your upper arm. Have a friend pump it up to about 200mm. of mercury, a sufficient pressure to stop the flow of blood to your arm. Initially your arm will feel an uncomfortable tightness beneath the cuff. Now comes the revealing part of the experiment: Perform any easy task with your cuffed arm. Merely flex your fingers and make a fist about ten times in succession, or cut paper with scissors, or drive a nail into wood with a hammer.
The first few movements seem quite normal at first as the muscles obediently contract and relax. Then you feel a slight weakness. Almost without warning, after perhaps ten movements, a hot flash of pain strikes. Your muscles cramp violently. If you force yourself to continue the simple task, you will likely cry out in absolute agony. Finally, you cannot force yourself to continue; the pain overwhelms you.
When you release the tourniquet and air escapes from the cuff with a hiss, blood rushes into your aching arm and a wonderful soothing sense of relief floods your muscles. The pain is worth enduring just to experience that acute relief. Your muscles move freely, soreness vanishes. Physiologically, you have just experienced the cleansing power of blood.