God calls the weak in order to demonstrate his power. He does not, yea, cannot, use those who believe all is done with their own resources and efforts. Like Joshua at Ai, he will work so that the victory is obviously his. His is the glory.
God creates situations and uses those chance moments that appear when man's powers are insufficient, so that observers must confess that it was he who acted. He looks for men and women who will allow him this freedom.
The great issue of the day is, will I allow him such freedom to act in my life — will I make myself weak, thereby creating space for his power to operate?