What is the full quote of “God is dead”?
Usually quotedGod is dead.
ActuallyGod is dead. God remains dead. And we have killed him. How shall we comfort ourselves, the murderers of all murderers?
Friedrich Nietzsche · The Gay Science (1882), section 125
Genuine — but it is spoken by a distraught character, and it is a lament, not a boast.
The three-word version is usually deployed as a triumphant announcement of atheism. In the text it is nothing of the kind. The words are spoken by a madman who runs into a marketplace carrying a lantern in the morning, looking for God, and is laughed at by the crowd.
What follows the famous line is a series of anguished questions about what happens next: who gave us the sponge to wipe away the horizon, whether we are falling in all directions, whether we must become gods ourselves simply to seem worthy of what we have done. This is a man describing a catastrophe, not celebrating a victory.
Nietzsche's point is that a civilisation has removed the foundation of its own values and has not noticed yet. The madman even says he has come too early. Quoting the first three words alone turns a warning about the difficulty of what comes after into a bumper sticker about religion.
For your English: Context changes speaker as well as meaning. In English, always ask who is speaking a quoted line — a narrator, an author, or a character the author invented in order to disagree with.