By whom was David taught
To aim the deadly blow,
When he Goliath fought,
And laid the Gittite low?
Nor sword nor spear the stripling took,
But chose a pebble from the brook.

'Twas Israel's God and King
Who sent him to the fight;
Who gave him strength to sling,
And skill to aim aright.
Ye feeble saints, your strength endures,
Because young David's God is yours.

Who order'd Gideon forth,
To storm the invaders' camp.
With arms of little worth,
A pitcher and a lamp?
The trumpets made his coming known
And all the host was overthrown.

Oh! I have seen the day,
When with a single word,
God helping me to say,
'My trust is in the Lord,'
My soul hath quell'd a thousand foes
Fearless of all that could oppose.

But unbelief, self-will,
Self-righteousness, and pride,
How often do they steal
My weapon from my side!
Yet David's Lord, and Gideon's friend,
Will help his servant to the end.

William Cowper

http://www.poemhunter.com/poem/olney-hymn-4-jehovah-nissi-the-lord-my-banner/