I once lived in Vancouver Canada. Above the city in the mountains there is a lake which serves as the city water supply. That lake is fenced, you can't swim in it, boat on it, or even hike around it. It's pure and will kept pure at all costs. To defile it would defile all.
What flows out of it can cleanse, quench and refresh but it stands alone. God like that lake is the only essence of purity in the universe. He stands eternally uncontaminated, pure and able to cleanse, quench and refresh. We have become contaminated by sin. Sin is both a moral and spiritual contamination. For us to enter into God in our state would forever contaminate the only one that can cleanse us. Jesus came to wash away our sin our contamination. His blood is the agent that can restore mankind to purity. The application of his atonement changes us from sinner to saint, from contaminated to pure.