Tawḥīd: Oneness
Everything else in Sufism sits on this one word. A heart runs on one center, and whatever sits there quietly sets the price of everything else. With two centers, choices start pulling apart, since what pleases one costs the other. The arrangement always breaks, because a day comes when only one of them can be obeyed.
Ikhlāṣ: Sincerity
Al-Ghazali gave sincerity a book of its own: one motive under an act, nothing else riding along. Most good deeds carry a passenger, some payoff the doer would hate to lose. A simple question exposes it: if the payoff disappeared tomorrow, would the act still happen? Someone serving one master answers yes without needing to think.
Shirk khafī: Hidden idolatry
Nobody signs up for hidden idolatry, which is why the tradition calls it hidden. Almost no one decides to put money in God's place; they check the balance first and pray afterward. A second master rarely asks for the title, only for the first look, the last thought, the thing a person would never risk. Whichever one gets that first look is the one being served.
Zuhd: Doing without
Sufis used the word for a loose grip on what you own. Wealthy people have had it and poor people have missed it, since the deciding thing is who you would still be if it all vanished. Someone able to picture that loss and still recognize themselves is holding it lightly. The money stays in the account, and it stops making the decisions.
Tawakkul: Trusting God to provide
Al-Ghazali treats reliance as resting the whole weight of your needs on God. Nobody keeps a second master for entertainment; the second one is insurance against the fear that the first will not come through when rent is due. Reliance does not hand over a plan; it drains the panic out of next month. When the fear goes quiet, the backup master loses the only job it ever had.