What is the Thing in A Wrinkle in Time, and which historical figures have opposed it?

The
Black Thing is never identified directly but it is defined indirectly through things related to
it and their antitheses. When Meg encounters IT, she learns IT is a means of telepathic mind
control. It overpowers Charles Wallace and keeps him captive on the planet when the others
tesseract and escape their captivity. After a near brush with the Black Thing, Meg returns to
rescue Charles. She succeeds in saving him through love since she has learned that communication
doesn't depend solely on rigidly correct formation of language.

The Black
Thing is thus identified as theof love and free, unrestrained human expression, i.e.,
communication. Those historical figures who have fought against The Black Thing and its minion
IT, were religious leaders, like Jesus, and philosophers, for instance,
Siddhartha.