From Holiness Day by Day: Transformational Thoughts for Your Spiritual Journey by Jerry Bridges:
To take up this sword [Ephesians 6:17], we must have it at hand, in our hearts. We must be like the psalmist who said, “I have stored up your word in my heart, that I might not sin against you” (Psalm 119:11). This principle of storing up God’s Word has a much wider application than only keeping us from sin. The Word, stored in the heart, provides a mental depository for the Holy Spirit to use to mediate His grace to us, whatever our need for grace might be.