The Mantra itself is Devatā, that is, the Supreme Consciousness (Cit–śakti) manifesting in that form.
śṛṇu devi pravakṣāmi bījānām deva–rūpatām |
mantroccārṇ amatrena deva–rūpam prajāyatte ||
Bṛhad-gandharva-Tantra (Ch. V)
The Mantra is awakened from its sleep (Mantra–caitanya) through the Sādhana Śakti of the mantrin. It is at base one and the same Śakti which appears as Sādhana–Śakti and Mantra-śakti, the latter however being the more powerful manifestation.
The consciousness of the Sādhaka becomes en rapport and in union with the Consciousness in the form of the Mantra; and the Devatā who is the Artha of the Mantra appears to the Sadhaka, whose mind has been cleansed and illumined by devotion.
Though the substance of the Mantra is Consciousness, that fact is not realised without the union of the Sādhaka’s Śakti derived from Sādhana with Mantra-śakti. The Devatā is then revealed.
In the case of Pūjā, Dhyāna and other Sādhanās, it is only the Sādhaka’s Sādhanā-Śakti which operates, whilst in the case of Mantra-sādhanā, Sādhanā-śakti works in conjunction with Mantra-śakti which is all powerful and re-inforces Sādhanā-śakti which is imperfect and meets with obstacles.
The individual Śakti is like fire. Just as waves of air, when struck and restruck by flames of fire, set up a blaze with redoubled force, so the Sādhaka’s individual Śakti when struck by Mantra-śakti is rapidly developed, and then a strong active individual Śakti unites with Mantra-śakti to make the latter doubly powerful.
It is because Mantra possesses this wonderful power that a Jīva can, it is said accomplish that which appears impossible. Otherwise a Jīva could not achieve by his own effort the treasure which is worshipped even by Śiva.
The Jaivī–Śakti or Śakti of a Jīva (as such) is transformed by the aid of Mantra into the Daivī-Śakti or the Śakti of a Deva (as such). With this Śakti he can accomplish that which a Deva can.
Mantra is thus an aspect of the Divine Mother appearing through Mantra-sādhanā with devotion to Her. It has been thus said (Tantra-Tattva II, 45):
“The string of fifty letters from A to Kṣa which are the Mātṛkās is Eternal, unbeginning and unending and Brahman itself.”
This great saying is the first of all Tantras. The realization of this Mantra-consciousness is Mantra-siddhi.
If a Sādhaka attains perfect Siddhi in even a single Mantra he becomes possessed of the spiritual knowledge which is acquired by learning all Vedas. For Veda is the Pariṇāma or evolution of the Dhvani of Kūla-Kuṇdalinī in the body of Īśvara, and the Pariṇāma of the same in the body of the Jīva is Śabda. For Veda is Dhvani uttered by Brahmā and Śabda is Dhvani uttered by the Jīva. In that Śabda is every form of Mantra which is that which gives vitality to the Jīva. It is this Dhvani too which evolves into gross Śabda as uttered sound, the body of the Devatā.
Siddhi in such Mantra is not gained so long as such Mantra is not awakened. The particular Mantra suitable for a Sādhaka is a matter determined by Cakra and other calculations.
Garland of Letters by Arthur Avalon