John Wesley on Numbers 19:17

Running water - Waters flowing from a spring or river, which are the purest. These manifestly signify God's spirit, which is oft compared to water, and by which alone true purification is obtained. Those who promise themselves benefit by the righteousness of Christ, while they submit not to the influence of his spirit, do but deceive themselves; for they cannot be purified by the ashes, otherwise than in the running water.