is,
A piece of spiritual armor.
God has given us armor to wear to protect us from the enemy and from our own temptations. We must be sure to put on our spiritual armor every day, just as the knights of old put on their armor before battle.

...And take the helmet of salvation, and the sword of the Spirit, which is the word of God...
Ephesians 6:17
(From Ephesians 6:10-18, encouraging us as Christian soldiers to put on "The Whole Armor of God".)

(See also, "...But put ye on the Lord Jesus Christ..." from Romans 13:14)