(I started writing this in April, but somehow ground to a stall, even though the ideas keep swirling. I don't think the ideas here are complete, but they're shareable, perhaps ingredients on which someone else can build.)
Theologically, Quakers are probably best known for their idea of Inner Light (or Inward Light), unmediated (though not necessarily easy!) contact with God, Christ, the Holy Spirit. The Light is Quakerism's positive message, a saving grace that heals, leads, improves, and grants the strength to achieve what might otherwise seem impossible.
At the same time, this Inner Light isn't the same as us. The Inner Light is not our personality or our desires and passions. We find it inside, but it isn't a part of us. For early Quakers, it was a help that might allow us to escape the evils in ourselves, as George Fox wrote in his Journal:
The Lord doth show unto a man his thoughts, and discovereth all the secret workings in man. A man may be brought to see all his evil thoughts and running mind and vain imaginations, and may strive to keep them down, and to keep his mind in, but cannot overcome them nor keep his mind within the Lord.
Now in this state and condition, submit to the spirit of the Lord, that shows them, and that will bring them to wait upon the Lord, and he that hath discovered them will destroy them.
Therefore stand in the faith of the Lord Jesus Christ, who is the author of the true faith, and mind him; for he will discover the root of lusts, and evil thoughts, and vain imaginations, and how they are begotten, conceived, and bred, and then how they are brought forth, and how every evil member doth work. He will discover every principle from its own nature and root.
So mind the faith of Christ, and the anointing which is in you to be taught by it, which will discover all workings in you, and as he teacheth you, so obey and forsake, else you will not grow up in the faith, nor in the life of Christ, where the love of God is received.
Now love begotteth love, its own nature and image: and when mercy and truth do meet, what joy there is! Mercy doth triumph in judgment; and love and mercy do bear the judgment of the world in patience. That which cannot bear the world's judgment is not the love of God, for love beareth all things and is above the world's judgment, for the world's judgment is but foolishness [...]
The fleshly mind doth mind the flesh and talketh fleshly. Its knowledge is fleshly and not spiritual, but savours of death and not of the spirit of life.
Now some men have the nature of swine wallowing in the mire, and some men have the nature of dogs to bite both the sheep and one another; and some men have the nature of lions, to tear, devour, and destroy. And some men have the nature of wolves, to tear and devour the lambs of Christ; and some men have the nature of the Serpent (that old adversary) to sting, envenom, and poison.
'He that hath an ear to hear, let him hear', and learn these things within himself. And some men have the natures of other beasts and creatures, minding nothing but earthly and visible things, and feeding without the fear of God. Some men have the nature of an horse, to praunce and vapour in their strength, and to be swift in doing evil; and some men have the nature of tall, sturdy oaks, to flourish and spread in wisdom and strength, who are strong in evil, which must perish and come to the fire.
Thus the evil is but one in all, but worketh many ways; and whatsoever a man's or womans nature is addicted to that is outward, the Evil One will fit him with that, and will please his nature and appetite to keep his mind in his inventions, and in the creatures, from the Creator.
Oh, therefore let not the mind go forth from God! for if it do, it will be stained, and venomed, and corrupted! And if the mind go forth from the Lord it is hard to bring it in again; therefore take heed of the enemy, and keep in the faith of Christ. Oh, therefore mind that which is eternal and invisible, and him who is the Creator and mover of all things! for the things that are made are not made of things that do appear; for the visible covereth the invisible sight in you. But as the Lord who is invisible doth open you by his invisible power and spirit, and brings down the carnal mind in you, so the invisible and immortal things are brough to light in you.
Oh, therefore, you that know the light walk in the light! for there are children of darkness, that will talk of the light and of the truth, and not walk in it. But the children of the light love the light, and walk in the light, but the children of darkness walk in darkness, and hate the light; and in them the earthly lusts and the carnal mind choke the Seed of faith; and that bringeth oppression on the Seed, and death over them.
Oh, therefore, mind the pure spirit of the everlasting God! which will teach you to use the creatures in their right place, and which judgeth the evil [...] So to live and walk in the spirit of God is joy, and peace, and life; but the mind going forth into the creatures, or into any visible things from the Lord, this bringeth death.
Now when the mind is got into the flesh and into death, then the accuser gets within, and the law of sin and death gets into the flesh. And then the life suffers under the law of sin and death; and then there is straitness and failings. For then the good is shut up, and then the self-righteousness is set a-top. And then man doth work in the outward law, and he cannot justify himself by the law but is condemened by the Light; for he cannot get out of that state but by abiding in the Light, and resting in the mercy of God, and believing in him, from whom all mercy doth doth flow. For there is peace in resting in the Lord Jesus [...] (Nickalls, 58-60, paragraph breaks added)
Quakers today seem rarely to speak of the darkness as bluntly as Fox did, though there are others who celebrate it (even religiously, to take an extreme example), and I often marvel at how selfish amorality can be celebrated by the "Greed is Good" side of American business culture.
The strength of the early Quakers seems to come from this contrast between the darkness they knew lurked in themselves and the guidance they received from the Light, from a God separate from themselves.