1 Thessalonians 5:8 (HCSB)

But since we belong to the day, we must be serious and put the armor of faith and love on our chests, and put on a helmet of the hope of salvation.

     In context Paul is referring to being prepared for the coming of the Day of The Lord. In contrast to those who are going about their lives as they want with no regard for God or for the return of Christ, we belong to the day—the light.  Arming ourselves only with faith and love, we are to hope in salvation and wait and watch patiently for it.