‘So then let us not sleep as do the rest, but let us watch and be sober, for those who sleep, sleep in the night, and those who are drunken, are drunken in the night, but let us, since we are of the day, be sober, putting on the breastplate of faith and love and for a helmet the hope of salvation.'
As sons of the day we are to watch and be sober, not sleep and be drunken (see Romans 13:13). The Christian does not ‘sleep' with regard to spiritual things (compare Mark 13:36; Ephesians 5:14), he studies them, keeps them in his heart, and lives them out in his life. He does not allow strong drink to dull his mind and hinder his witness, frittering away his life in idle pleasure. Instead of being drunk with wine he is filled with the Spirit (Ephesians 5:18-19).
He puts on the breastplate of faith and love and the helmet of the hope of salvation. The switch to armour may indicate the idea of a soldier armoured and on watch, in contrast with his mates who are off duty and drunk or asleep. And the Christian does this by responsive faith and ‘putting on love'. His Spirit-produced love for his brothers and sisters, and for his neighbour, the result of worship, meditation on God's word and praise from the heart, and the working of God within, keeps his heart protected from evil and the Evil One, and his mind is concentrated on the hope of final salvation, when he will be made like Christ, for he will see Him as He is (1 John 3:2), and this involves looking actively for the coming of Christ (Luke 12:35-40).
This ‘hope of salvation' is described in many ways. See Romans 8:23-24; 1 Corinthians 15:42-49; 2 Corinthians 5:1; Galatians 6:8; Ephesians 1:4; Ephesians 5:27; Philippians 3:21; Colossians 1:22; 1Th 3:13; 2 Thessalonians 2:14; Hebrews 12:22-23; 1 Peter 1:4; 1Pe 5:10; 2 Peter 3:13; Jude 1:24; Revelation 21:1 to Revelation 22:5.
In contrast the unbelievers are asleep in the night, and drunken in the night. They are not living in readiness for ‘the day' for they do not believe in the day. They are not waiting and watching, they are spiritually asleep and drunk.
For the use of armour to depict spiritual virtues and activity see Romans 13:12; 2 Corinthians 6:7; 2 Corinthians 10:4; Ephesians 6:13-18 compare Isaiah 59:17.