In Ray Bradbury's Fahrenheit 451, there are many literary and biblical allusions. At the novel's end, after Montag has crossed the river, he listens as others like him discuss having memorized entire books. Montag tries to remember things he has memorized, and one of those is a line from the Bible, specifically Revelation 22:2.
First of all, "Revelation" speaks to a "revelation," when God will show himself and his will to his people. The book of Revelation is also referred to as apocalyptic literature. The association of the book's intent is to provide a description of the "end times" and the promise of life afterward, as predicted in its chapters.
...the genre is known as apocalyptic literature. Such literature is 'marked by distinctive literary features, particularly prediction of future events...
In the scriptural passage noted, there is mention of "the tree of life."
The tree of life may symbolize the future of mankind: the book people who have escaped to the other side of the river. The fruit may be symbolic of the knowledge these people possess, which will nourish a new society. The leaves are said to be specifically for the "healing of the nations." As devastating bombings have occurred where Montag lives, and in other cities across the country, we can assume that the only hope for civilization is that it be rebuilt: specifically, Revelation refers to "healing"—we can assume of the land, the society and its people—the verse refers to the nation, which would be all of these things.
Whereas Revelation speaks to "reaching the city," (the "holy city of God") or Heaven, in this story, it alludes to a new life—one very different from that which these people have left: the society they have witnessed being destroyed. There is also a sense of resurrection: Revelations refers to it in a spiritual sense, but Bradbury may refer to a resurrection of not only Montag, but also of mankind:
'When we reach the city', the last line of the book, contains a solid link between Montag’s destructive world and the Apocalypse of the Bible, thus his spiritual resurrection.
When Montag says he'll "save it" for when they reach the city, he means he will speak of it when they arrive at their destination: where they will begin to rebuild society based on the knowledge they all have gathered collectively. Montag is delivered from destruction, and Bradbury leaves the reader with "a tinge of long-term hopefulness."