From Charles H. Spurgeon:
"For my strength is made perfect in weakness."
2 Corinthians 12:9
A primary qualification for serving God with any amount of success, and for doing God's work well and triumphantly, is a sense of our own weakness. When God's warrior marches forth to battle, strong in his own might, when he boasts, "I know that I shall conquer, my own right arm and my conquering sword shall get unto me the victory," defeat is not far distant. God will not go forth with that man who marches in his own strength. He who reckoneth on victory thus has reckoned wrongly, for "it is not by might, nor by power, but by my Spirit, saith the Lord of hosts." They who go forth to fight, boasting of their prowess, shall return with their gay banners trailed in the dust, and their armour stained with disgrace. Those who serve God must serve him in his own way, and in his strength, or he will never accept their service. That which man doth, unaided by divine strength, God can never own. The mere fruits of the earth he casteth away; he will only reap that corn, the seed of which was sown from heaven, watered by grace, and ripened by the sun of divine love. God will empty out all that thou hast before he will put his own into thee; he will first clean out thy granaries before he will fill them with the finest of the wheat. The river of God is full of water; but not one drop of it flows from earthly springs. God will have no strength used in his battles but the strength which he himself imparts. Are you mourning over your own weakness? Take courage, for there must be a consciousness of weakness before the Lord will give thee victory. Your emptiness is but the preparation for your being filled, and your casting down is but the making ready for your lifting up.
"When I am weak then am I strong,
Grace is my shield and Christ my song."