Greatest and most fundamental principle of the Baha'i Faith

"And now, one of the greatest and most fundamental principles of the Cause of God is to shun and avoid entirely the Covenant-breakers, for they will utterly destroy the Cause of God, exterminate His Law and render of no account all efforts exerted in the past."

Second section of the Will and Testament of Abdul Baha
Ruhi Book 8, Unit 1 (March 2012)

http://bahai-library.com/pdf/ruhi/ruhi8a.pdf