With regard to sin, I think that in Japanese it is called "tsumi". and I think the concept of purification was explained in several discussions that the goal is to accomplish one's own mission in life, and to help others, who may have strayed from their own individual missions in life. There is also a concept which was explained something like the phrase "ha wo migaku" polishing or brushing one's teeth. The sword is also polished, the mirror is also polished, all a similar concept. Life can be a polishing process to improve us. These are some concepts that I have heard applied to the martial arts, sorry if my memory of Japanese language isn't very accurate.
I love the analogy of the polishing process. That is a very apt description and metaphor for the benefits of the martial arts. I often think of the traditional katana making process when I read in Scripture that "our God is a consuming fire
". I think about the metal being heated and folded and the process being repeated dozens of times in order to produce a blade of magnificent quality and durability. God purges the dross from our lives so that we are like purified metal. If that process doesn't take place then we are useless or weak and easily broken. Sometimes that process is painful - like the white hot heat of the furnace - but it is ultimately for our good. When I'm at AJJ and a technique is applied and I feel the pain in my joint or body part, I often think about how it is 'good' pain, a pain that produces wisdom and character.
Scripture itself is described as a 'mirror' to the soul (James 1:22-25), by allowing the Scriptures to permeate our lives and change us from within, we will grow in Christ-likeness and godliness. I've heard the analogy been used before for Zen of the moon and the lake. When the lake is still, the moon's light and image can be reflected perfectly off the water. Sometimes I appropriate that analogy for Christ - Jesus is the lake and God is the moon; Jesus is the perfect reflection of God the Father. As we learn about Christ through the Scriptures (and as a Christian I believe the Bible is the inspired Word of God), it has presevered unerringly the life and teachings of Jesus. Thus the Scriptures themselves act like the water, to continue the metaphor. As we meditate on the Bible and allow it to act like that mirror in our lives we can permit God to purge the sinful nature and purify us into a closer reflection of Him.