” Every institution has its letter as well as its spirit, as every word must refer to something of which it is the sign or significator. The Gospel has both its letter and its spirit; and multitudes of professing Christians, by resting in the Letter, receive not the life which it is calculated to impart. Water, in baptism, is the letter that points out the purification of the soul; they who rest in this letter are without this purification; and dying in that state they die eternally. Bread and wine in the sacrament of the Lord’s Supper, are the letter; the atoning efficacy of the death of Jesus, and the grace communicated by this to the soul of a believer, are the spirit. Multitudes rest in this letter, simply receiving these symbols, without reference to the atonement, or to their guilt; and thus lose the benefit of the atonement and the salvation of their souls. The whole Christian life is comprehended by our Lord under the letter, Follow me. Does not any one see that a man, taking up this letter only, and following Christ through Judea, Galilee, Samaria, etc., to the city, temple, villages, seacoast, mountains, etc., fulfilled no part of the spirit; and might, with all this following, lose his soul? Whereas the Spirit, viz. receive my doctrine, believe my sayings, look by faith for the fulfillment of my promises, imitate my example, would necessarily lead him to life eternal. It may be safely asserted that the Jews, in no period of their history, ever rested more in the letter of their law than the vast majority of Christians are doing in the letter of the Gospel. Unto multitudes of Christians Christ may truly say: Ye will not come unto me that ye may have life.”
From Adam Clarke’s Bible Commentary , on 2 Cor 3: 6.