This section is from the book "Karpuradistotram. Karpuradi Stotra Tantrik Texts", by Arthur Avalon (Sir John Woodroffe). Also available from Amazon: The Serpent Power: The Secrets of Tantric and Shaktic Yoga.
In the Satyayuga Brahma revealed the Smritis, Puranas and other Shastra to the Devarshis. In this way Brahmavidya was promulgated to the world. This therefore is authority to show, that just as Brahman is everlasting, so are the Agamas and Nigamas which tell of Brahman. Just as in the Satya and other Yugas, only the three twice-born castes, wearing the sacred thread, but not the Shudra and other low castes were entitled to worship according to the Veda, so in those three Yugas only Devarshis, Brahmarshis and Rajarshis, who had conquered their passions and knew Advaita doctrine and Brahman, were entitled to the Agama Shastra which destroys all sense of difference caused by ignorance and grants knowledge of Advaitatattva.
By Shiva's command they kept it as secret in their heart as they would a knowledge of their own mother's illicit love. By Upasana they became liberated whilst yet living (Jivanmukta) and attained to Brahrnanirvana. At that time the Upasana of the Agama was unknown to Sadhakas devoted to Karma. For this reason many people nowadays think the Tantrashastra to be of recent origin. Probably all know that in the first three Yugas Brahmana boys, after investiture with the sacred thread, used to learn the Karmakanda and Jnanakanda of the Veda orally from their preceptors. The Veda was not then classified or reduced to writing. Towards the close of the Dvaparayuga, Shrikrishna-dvaipayana Maharshi Vedavyasa divided the Veda into four parts and reduced it to writing. This however does not show that the Veda is a recent production. The Supreme Science (Para vidya) which is contained in the Agama was also handed down from generation to generation of Gurus in the first three Yugas and is being now similarly transmitted. Towards the end of the Dvaparayuga, and at the beginning of the Kali age, merciful Shiva impelled by compassion for humanity bound in the toils of ignorance, divided the Tantrashastra, which is unlimited knowledge, into three sets of sixty-four parts each, according to the necessity of different Adhikaris, and then told them to Gana-pati and Kartikeya the two beloved sons of Parvati. They repeated these Tantras to Rishis of Siddhashramas, and these last, in their turn, told them to their own disciples. Of the Rishis who knew Agama the chief was Dattatreya, an incarnation of Vishnu. At the beginning of the Kalpa the ancient Brahmavidya contained in the Agama appeared from out the Parameshtiguru who is Mahabrahmavidya and exists in man's heart unlimited and imperishable. If Sadhana is done according to the instructions of a Sadguru it becomes visible in the Sadhaka's heart. Upasana, in the Vaidik form, predominated in the Satyayuga. In those days Brahmanas, and other twice born castes, impelled by a desire for wealth, progeny and so forth used to worship Indra, Agni, Vayu, Suryya, Soma, Varuna and other Devas presiding over particular Shaktis of Parameshvara in whom all Shaktis reside. But desire-free Brahmarshis and Maharshis did Sadhana, of Brahmavidya the full and perfect Shakti. And so we see in the tenth Mandala of the Rigveda Samhita, that Mahadevi appeared in the heart of the daughter of Maharshi Ambhrina and so told the true nature of Brahmavidya to Rishis. This is the Devisukta full of Advaitatattva, the Hymn telling of the true nature of Brahmavidya in the Veda. In the Treta and other Yugas the Brahmanas and other twice-born devoted to the Karma-kanda used to perform Yajnas and so forth, according to the Smritishastras of Manu and others. But Brahmarshi Vashishtha (in Chinachara) Rajarshi Vishvamitra" (see Gandharva Tantra, First Patala), Videharaja Janaka, Bhrigurama the son of Jama-dagni (see Kalikulasarvasva), Shri Bamachandra and other high souled men were worshippers of Brahmavidya the full and perfect Shakti. Again in the Dvaparayuga, despite the existence of Vaidik and Smarta cults, the Agnihotra Yajna and other rites used to be performed according to the Puranas. But high-souled Shrikrishna the son of Vasudeva (see Radha, Tantra, Devi Bhagavata and Mahabharata Anushasana Parva, Oh. 14), the five Pandavas namely Yudhishthira and others (Virata Parvva, Ch. 6) the high souled Rajarshi Bhishma, the great Muni Vedavyasa, high souled Shukadeva, Asita, Devala and Brahmarshis :such as Durvasa were worshippers of Mahavidya the perfect Shakti. Of this the Mahabharata and other books provide particular proofs. In the present Kaliyuga also the ten Sangskaras such as marriage and so forth of the twice born, and the obsequial ceremonies such as Shraddha are performed according to Vaidik ritual. Smriti governs Chandrayana and other matters relating to Ashrama and legal affairs such as inheritance. The autumnal Durgapuja, and other Vratas are performed according to the Puranas. But initiation, Upasana of Brahman with Shakti and various practices of Yoga are done according to the ritual of the Agama Shastra.
This latter is of three kinds according to the prevalence of the Gunas namely Tantra, Yamala and Damara. There are in all 192 Agamas current, namely 64 each in Ashvakranta, Ratha-kranta, and Vishnukranta. Many Tantras were lost in Buddhist and Mahommedan times and the few which still remain with Sadhakas in different parts of the country are not shown by them to any but to their disciples, so that these also are about to be lost. The late Rasika Mohana Chattopadhyaya, with great effort and cost, saved some of these and the English Arthur Avalon has done the same and I hope yet others will in future be rescued by him.
In the Yogini Tantra ishvara says to Devi that the difference between Vedas and Agamas is like that between Jiva and Atma, that is between Jiva covered with Avidya and Ishvara who is full of Vidya. Indra and other Devas who used to be worshipped as Ishvaras in Yajnas held under the Karmakanda or Samhita of the Vedas are, in Tantra-shastra, worshipped as the Presiding Devatas of the Dikpalint Shakti of Her who is all Shaktisvarva-shaktisvarupinl). The three ishvaras Brahma, Vishnu and Rudra of the Vedas and Purauas are in Tantra-shastra the presiding Devatas of the creative, preservative, and dissolving Shaktis of Mahadevl. As such they are worshipped as the supports of the couch of the Mahadevi. She in the Devigita says that ' Brahma, Vishnu, Ishvara, Sadashiva are five Mahapreta at my Feet. They are constituted of the five Bhuta and represent the five different elements of matter.' ' I however ' She says 'am unmanifested consciousness (Chit) and in every way beyond them.'