How is the line, "For the word 'We' must never be spoken, save by one's choice and as a second thought" related to Anthem's theme?
This is a key quote to the theme of Anthem, because it comes at Equality 7-2521's (now Prometheus's) revelation that he is, in fact, his own person. He can now call himself "I" when he speaks to Gaea and anyone else he is able to extricate from the World Society. It is not until the last chapter that he realizes this fact. He learns it from reading all the books that are in the house on the summit that ends their journey to safety.
To give some perspective on this passage, we can look at the point in the novel when Equality 7-2521 attempts to bring light to the World Council. They are at first puzzled by his discovery, saying: "What is not thought by all men cannot be true." They then ask Equality 7-2521 whether he worked on the light alone. When he says yes, they say, "What is not done collectively, cannot be good."
In the World Society, "I" is the bad word, the forbidden word. Equality 7-2521 must refer to himself as "We." Rand makes the point at the end of the book that groupthink (thinking collectively as a group) is harmful and that individuals must be able to express themselves for a society to advance.