The letter Mem [מ] symbolizes water and is associated with the number forty. If that seems an odd pairing, we should consider that the first “forty” in scripture denotes the duration of the Genesis flood in days. Thereafter, the number is often associated with testing or judgment.
In the New Testament, water is most frequently associated with the Holy Spirit of God. It is not without reason that we call that great, singular event in which the Spirit came to indwell all who are in Christ and bind together Jew and Gentile into one body a “baptism”. But water serves other purposes than cleansing and testimony. It meets the perpetual need of humanity. Jesus cried out to the thirsty, “Come to me and drink.” John comments, “He said this about the Spirit.”
How does the Spirit operate in the human heart? Well, he uses the word of God, which is where our psalmist comes in once again.