Fernand Prat on εν ρηματι ("by the word") as a reference to the Essential Formula/Wording of Baptism in Ephesians 5:26

 

The only admissible meaning, therefore, is: “By purifying it by the baptism of water (instrumental dative) by means of a word” (εν instrumental).—The baptismal water alone has not the power to purify the soul; there must be added to it “a word,” which is indeed the formula of baptism, although it is designated here only generically . . . (Fernand Prat, The Theology of Saint Paul, 2 vols. [trans. John L. Stoddard; Westminster, Md.: The Newman Bookshop, 1927], 2:256 n. 2)