Believers belong to the day and must continually be on guard. Paul uses the image here of a Roman soldier (see 2 Corinthians 6:7; 2 Corinthians 10:4; and especially Ephesians 6:13–18). The soldier has girded himself with armour and has placed a protective helmet on his head.
A Christian knows the weapon(s) of faith and basks in God’s love and esteems himself rich in the salvation, given by God in Jesus Christ. In Romans 13:14 the apostle Paul summarizes it beautifully by saying put on the Lord Jesus Christ.
8 But since we belong to the day, let us be sober, having put on the breastplate of faith and love, and for a helmet the hope of salvation.