Baha'u'llah Says :
"Protect yourselves with utmost vigilance, lest you be entrapped in the snare of deception and fraud." This is the advice of the Pen of Destiny."Therefore, to avoid these people will be the nearest path by which to attain the divine good pleasure; because their breath is infectious, like unto poison.""Endeavor to your utmost to protect yourselves, because Satan appears in different robes and appeals to everyone according to each person's own way, until he becomes like unto him -- then he will leave him alone."
"...If you detect in any man the least perceptible breath of violation, shun him and keep away from him." Then He says: "Verily, they are manifestations of Satan."
(Bahá'í World Faith, p. 431)
Abdol Baha Says :
Bahá'u'lláh, in all the Tablets and Epistles, forbade the true and firm friends from associating and meeting the violators of the Covenant of His Holiness, the Báb, saying that no one should go near them because their breath is like the poison of the snake that kills instantly.
(Bahá'í World Faith, p. 430)
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.
(Bahá'í World Faith, p. 448)
My object is to show that the Hands of the Cause of God must be ever watchful and so soon as they find anyone beginning to oppose and protest against the guardian of the Cause of God, cast him out from the congregation of the people of Bahá and in no wise accept any excuse from him. How often hath grievous error been disguised in the garb of truth, that it might sow the seeds of doubt in the hearts of men!
(Will and Testament of Abdu'l-Bahá, p. 12)