Tantra has some important opinions on the power of Mantra. For example, ordinary people think that “Om, Vajrasattva hūm” is just Vajrasattva mantra, by its very nature, just a few words or sounds made from one’s throat, it is not Buddha. And Vajrasattva only exists when there is an image or a statue. In fact, for ordinary people, Vajrasattva will not preach them directly as people do not have such karmic reward. Only first-stage Boddhisattva or above, can get escorts by Vajrasattva directly. However, if we have enough confidence on Vajrasattva, the Mantra itself is Vajrasattva. Having realized that the Mantra and the actual Vajrasattva are with no difference, a single “Om, Vajrasattva hūm” will have huge merits.
[Excerpt from Luminous Wisdom Book Series: The Blessing of Speech]