In I John 1:7, the apostle writes that we are cleansed by the blood of the Lamb. But the cleansing found here in Ephesians 5:26 is of a different kind. Hebrews 9:22 says, "Almost all things are purged by blood." Almost all but not everything is. There are some things that must be purged in another way.
Ephesians 5:26 tells us that we are cleansed "with the washing of water by the word." There are things that will be cleansed—things in our minds, things that deal with conduct, things that have to do with character and attitude—that are cleansed by water. The word "water" here is symbolic, referring to the Word of God, as well as to the Holy Spirit.
Christ gave a long discourse in John 6, which we often apply at Passover time, about eating His flesh and drinking His blood. Towards the end, He says to his audience, "The words that I speak to you, they are spirit, and they are life" (John 6:63).
We have in the Bible the Word of God—and Jesus says it contains power. It has power to cleanse a person's mind, because we can think only by what goes into the mind, concepts that are contained in words. Words are merely symbols of ideas that we use to reason. We turn those ideas into action, into conduct, which becomes part of our character and our attitude.
In other places in the Bible, the Holy Spirit is compared to water and to oil. Both of these have revitalizing, nourishing, cleansing, purifying, and sanitizing properties to them. We are familiar with how we use water to cleanse things; water is the universal solvent. We do not use oil so much to cleanse things, but, on the other hand, the Samaritan in the parable treated the man's wounds with oil (Luke 10:34). It had a purifying effect on him.
Therefore, we are washed by the water of the Word of God in conjunction with a new nature that is given to us by God through His Spirit. This begins to help us to understand why studying the Word of God is so important. We need those words in us so that we can think according to them, and if we believe those words, they will begin to purify and cleanse the way we think.