In A Wrinkle in Time, how are people like Pasteur and Gandhi "lights for others to stand by"?

After Mrs.
Whatsit, Mrs. Which, and Mrs. Who show the children the "Dark Thing" covering the
Earth, this leads to a discussion of evil. The children are distressed about the evil on Earth,
and Meg asks if the "Dark Thing" has just arrived there.

Mrs.
Whatsit explains that evil has been on the planet for a long time, but she also offers hope. She
tells the children that many people have fought and continue to fight the darkness of ignorance
and violence. These include Jesus, but the fighters in the grand and "exciting" battle
between good and evil also include Pasteur and Gandhi. Pasteur used science and reason for the
good purpose of fighting disease, and Gandhi used his philosophy of...