The Apostle Paul is saying to the church in Corinth (and to us), that hearts can turn from dark to light (from flesh to Spirit) by accepting Jesus as Lord and Savior. When God said “Let there be light”, He created all light. The sun in the sky is one of many ‘out there’, but the light that God places inside each of His believers is the greatest and most powerful light of all.
The very Light of the World is available to each of us! To have the Spirit of God take up residence inside the heart of simple “jars of clay” like us (aka frail and broken humans) is truly a staggering thing to consider! And it is truly the greatest treasure that we could possibly possess.
The only thing you and I have right now that is built for eternity is our spirit. This is where the Holy Ghost does His ‘haunting’… And it makes sense, doesn’t it? Why would an Infinite Being take up residence in a temporary shelter?
But be aware that He is also a gentleman – He will not force Himself on anyone that doesn’t want Him there… And it's sad to say, but if you do refuse His advances please understand that you will have no excuse!
For since the creation of the world God’s invisible qualities—his eternal power and divine nature—have been clearly seen, being understood from what has been made, so that people are without excuse. (Romans 1:20)
Please accept Him into your life, and let Him live in you. I can speak from personal experience that it is the best Gift you will ever receive!