Master morality begins with a self-affirmation. The noble type calls itself "good" — strong, creative, proud — and defines "bad" only as a secondary concept, meaning roughly "lesser" or "weak." The master does not look outside himself for validation; his values flow from an inner abundance.
Slave morality begins differently — with a "no" to the powerful other. Unable to overcome through direct action, the weak redefine strength as evil and weakness as virtue. Humility, meekness, pity — these become good not because they are affirming but because they invert a power relation that the weak cannot otherwise overcome.
For Nietzsche, Christianity is the most successful example of this inversion. "The meek shall inherit the earth" is not an innocent promise; it is the revenge of the powerless against the powerful, dressed in the language of divine will.
Nietzsche does not straightforwardly endorse master morality as a return to some earlier ideal. He recognizes that the slave revolt produced enormous cultural complexity. His target is not strength but authenticity — the refusal to dress one's actual motivations in false clothing.