Charles Hodge on Ephesians 6:15

V. 15. In ancient warfare which was in a large measure carried on by hand-to-hand combats, swiftness of foot was one of the most important qualifications for a good soldier. To this the apostle refers when he exhorts his readers to have their feet shod, ἐν ἑτοιμασίᾳ τοῦ εὐαγγελίου τῆς εἰρήνης, with the preparation of the gospel of peace. According to one explanation εὐαγγελίου is the genitive of apposition, and the Gospel is the ἑτοιμασία with which the Christian is to be shod. Then the idea is either that the Gospel is something firm on which we can rest with confidence; or it is something that gives alacrity, adding as it were wings to the feet. Others take εὐαγγελίου as the genitive of the object, and ἑτοιμασία for readiness or alacrity. The sense would then be, ‘Your feet shod with alacrity for the Gospel,’ i. e. for its defence or propagation. The simplest interpretation and that best suited to the context, is that εὐαγγελίου is the genitive of the source, and the sense is, ‘Your feet shod with the alacrity which the Gospel of peace gives.’ As the Gospel secures our peace with God, and gives the assurance of his favour, it produces that joyful alacrity of mind which is essential to success in the spiritual conflict. All doubt tends to weakness, and despair is death.