"To them that have no might he increaseth strength." Isaiah 40:29
The Lord's people are often in this state, that they "have no might." All their power seems exhausted, and their strength completely drained away; sin appears to have got the mastery over them; and they feel as if they had neither will nor ability to run the race set before them, or persevere in the way of the Lord. Yet, even then, they have strength; for it says, "he increaseth strength." It does not say, 'he gives, bestows, communicates strength;' but "he increaseth strength."
How can this be?
We must have power to feel our weakness. God must put forth his power to enable us to fall down into nothingness and helplessness. It therefore says, "he increaseth strength." As though it would imply, 'Is not the very power to sink down into creature weakness, helplessness, and nothingness, strength?' It is so in God's mysterious dealings. And, therefore, "to them that have no might" (in other words, those who are sensible in their own consciences that they have no power at all, who are completely exhausted of nature's strength and wisdom), to these "he increaseth strength."
Now the Lord "increaseth strength" in a very mysterious way. He often drops strength stilly and secretly into the soul. We are not always to expect very great manifestations. This is not the way in which the Lord usually increases strength. His visits to the soul are often better known by their fruits and effects, and by looking back upon them when they are past, than by any immediate impulse. The strength given is more easily felt than the hand seen which communicates it. In this respect it much resembles the new birth, of which the Lord says, "The wind bloweth where it listeth, and thou hearest the sound thereof, but canst not tell whence it cometh, and whither it goeth" (John 3:8).