Hello. From the background of your question, we can see that you are referring to the book "Fahrenheit 451," which tells the story of a society dominated by an authoritarian government that prohibits the reading of books. In this society, firefighters are responsible for burning books and the houses where those books are found.
Montag, quoted in the question above, is one of those firefighters. However, little by little he starts to get in touch with the books and realizes how empty his life is without reading. In this case, Montag joins a group of people who are fighting to overthrow this government and allow reading to be a legal activity again.
When Montag says the line "It was not burning, it was warming," he is referring to how fire has the ability to bind as well as destroy. That's because he had just run away from a chase, until he found a group of teachers around a campfire. As a firefighter, Montage had only seen the destructive power of fire, but there, he saw how fire plays a role in bringing people together, making them comfortable and allowing a positive and meaningful conversation to be established between people. This quote shows that everything that is destructive can be reframed by people with good knowledge.
