Utterance must needs possess ....

O My Name! 
Utterance must needs possess penetrating power. For if bereft of this quality it would fail to exert influence. And this penetrating influence dependeth on the spirit being pure and the heart stainless. Likewise it needeth moderation, without which the hearer would be unable to bear it, rather he would manifest opposition from the very outset. And moderation will be obtained by blending utterance with the tokens of divine wisdom which are recorded in the sacred Books and Tablets. 

Thus when the essence of one’s utterance is endowed with these two requisites it will prove highly effective and will be the prime factor in transforming the souls of men. This is the station of supreme victory and celestial dominion. Whoso attaineth thereto is invested with the power to teach the Cause of God and to prevail over the hearts and minds of men.

 Bahá’u'lláh