“The greatness of the man’s power is the measure of his surrender.”
― William Booth
Being that William, an English Methodist preacher, along with his wife Catherine founded the Salvation Army, I would contend that “power” in this context aligns in part with Merriam & Webster: “the ability to act or produce an effect; specifically : an ability that seems extraordinary or superhuman.” Think about what they together created and shared with the world.
Yet I suspect that Mr. Booth and his wife were touched more by the power of the Holy Spirit, choosing to pursue a godly life, embracing and enduring hardship, and performing selfless acts of service, wonder and love.

That is the power that can only be discovered in surrender. Full out. Though at first it may feel like all you can do is dip your toe in it, there is nothing but merit to be realized once you just jump in.
If you think about it, more often than not, in a sense, you have already sunk. There is not much of an existence to be had living on the bottom. Being fully surrendered brings you up, back to the top, ready to be in the light and live as you were meant to live.
Salvation as it were.
Part of a loving army of men and women pursuing a godly life, embracing and enduring hardship and performing selfless acts of service, wonder and love.
As an enlistment with the Booth’s cause.
And with Him.