Qalb: The heart, the center where a person thinks, wants, and decides
Al-Ghazali uses heart for the whole inner center, mind and will included. That matters because a person can agree with something in their head while their wanting runs off somewhere else. Loving God with everything means those parts stop pulling against each other and face one direction. The whole inside is being asked to move together.
Maḥabba: Love
Al-Ghazali places love of God near the very end of the Revival, as the point everything before it serves. Duty and fear can carry a person a long way, though both keep a distance, and both leave room for bargaining. Love closes that distance, so the same acts get done for the one they are for, not for the credit. When love is the reason, the question of how little you can get away with stops coming up.
Ikhlāṣ: Sincerity
Ikhlas names a motive with nothing else stirred into it. Most good deeds come mixed with other wants: to be seen, to feel safe, to be thought well of. Al-Ghazali says the mixing happens so quietly that a person can give generously and never ask who the gift was for. Keeping one reason leaves the act whole, instead of partly for God and partly for the room.
Neither their flesh nor their blood reaches God, but your reverence does: the offering carries only what your heart puts in it
The Quran says this about animals brought for sacrifice, and Sufis return to it whenever religion starts feeling like a transaction. Nothing is wrong with the offering; the trouble starts when a person believes the object does the work. A costly gift can be handed over while the giver stays exactly as they were. Only the state the person was in makes it across, and the rest stays behind.
None of you believes until he loves for his brother what he loves for himself: your own wanting sets the measure
The Prophet gave a plain gauge for how much you owe the people near you. Wishing everyone well in general costs nothing, and most of us do it without thinking twice. This asks something narrower: the same close care you spend on your own comfort, safety, and good name, pointed at someone else. When that happens, caring about God and caring about the person in front of you stop being two separate jobs.