From “Joyful Notes in the Strife of Suffering,” within Christian Discourses:

That God is the strongest is evidenced by one sign, that man is entirely weak. For God there is but one obstacle, man’s selfishness, which steps in between God and man like the shadow of the earth when it produces the eclipse of the moon. If there is this selfishness, then man is strong, but his strength is God’s weakness; if the selfishness is absent, then man is weak, and God strong; the weaker man becomes, the stronger God becomes.

However, if this is so, then in another sense, in the true sense, the relation is inverted; and with this we come to the joyfulness.

For he who is strong without God, precisely he it is who is weak. The strength by which a man stands alone without God is weakness. God is in such a degree the strong one that He is all strength, is strength itself. So to be without God is to be without strength. So to be strong without God is to be strong…without strength; it is like being loving without loving God, and so to be loving without love, for God is love.

…Is not this then joyful, that the weaker thou dost become, all the stronger does God become in thee, or is it not joyful that thou dost become weak? Is there at bottom anything to complain of because a hard lot fell to thee, that perhaps which thou didst most dread and which made thee entirely impotent and weak? The weaker thou dost become, so much the stronger does God become in thee. And that this is joyful thou thyself wilt surely admit! Consider how poor a man would be if he could pass his whole life, proud and self-satisfied, without ever having admired any one; but how horrible a thing if he could pass his whole life without ever having marvelled at God, without ever having lost himself in worship through marvelling at God! But one can worship only by becoming oneself weak, thy weakness is essentially worship; woe to the presumptuous man who in his presumptive strength would be audacious enough as a strong man to worship God! The true God can be worshipped only in spirit and in truth–but precisely this is the truth, that thou art entirely weak.

So then there is nothing in the world to fear, nothing to fear in that which can bereave thee of all thine own power and make thee entirely weak, which can break all thy confidence in thyself and make thee entirely weak, which can completely sap thy earthly courage and make thee entirely weak–for the weaker thou dost become, all the stronger does God become in thee.