and take the helmet of salvation, and the sword of the Spirit, which is the word of God, 18 praying at all times in the Spirit, with all prayer and supplication. To that end keep alert with all perseverance, making supplication for all the saints, – Ephesians 6:17-18
The conflict which the apostle has been speaking about is not just a single battle between the individual Christian and Satan, but also a war between the people of God and the powers of darkness. No soldier entering battle prays for himself alone, but for all his fellow soldiers also. They form one army, and the success of one is the success of all. In a similar way Christians are united as one army and therefore have a common cause, and each must pray for everyone else. Such is the communion of saints, as set out in this letter and in other parts of Scripture, that they can no more fail to take this interest in each other’s welfare than the hand can fail to sympathize with the foot.
Crossway Classic Commentaries – Ephesians (Wheaton, IL; Crossway Books; 1994) Commentary on Ephesians 6:18