Tantra is often seen as non-Vedic, even as a revolt against the Vedas in Hinduism by some scholars. Part of this is internal to the tantras themselves: certain tantras exult in independence from Vedic tradition, exult in shocking the orthodox, though many others, like the Mahanirvana Tantra, see themselves as integral parts of the Vedic tradition. And part of it is external, traceable to Western scholars who excel at analysing, separating, and isolating into impermeable categories.
But are the Hindu tantras really anti-Vedic or even non-Vedic? An objective examination shows that they are not at all. There might have been non-Vedic influences, certainly, but the core of tantric thought and practice is clearly traceable to the Vedas. And a very strong case can be made that the Hindu tantras are an important part of that effort to bring India back to its original Vedic trajectory.
First of all, the Atharva Veda seems to have been a major source of tantric traditions, and there are links with the other three Vedas as well. Secondly, much of what we see in the tantras is a reinterpretation of Vedic symbolism. This is important.
The symbolism which was alive and vital during the Vedic period was largely lost afterwards. A new symbolic world was born with the Puranas and Itihasas, starting before the birth of Buddhism and continuing to the present day. But what do we find in the tantras, more than in the Puranas and Itihasas? A reinterpretation of Vedic symbolism.
The ritualism of the karmakanda, the section of the Vedas dealing with rituals, whose symbolism was becoming obscure by the time of the Buddha, was replaced by a tantric ritualism where the spiritual symbolism was transparent. The tantric homa fire, ritually identified with the deity being worshipped, replaced the Vedic fire. Yantras replaced the sacred geometry of the altars. Images replaced the Vedic deities worshipped through the medium of Agni, the fire god. The worship was developed as a means for making real and tangible the teaching, ‘Thou art That’ – a means, in fact, for driving that teaching of identity into the very nerves and blood.
The different disciplines of the Vedas and Upanishads, whose method of practice was lost, were replaced by various types of tantric worship and meditation. Even the Vedic sandhya-vandana, twilight devotions, was replaced by a tantric version. The Vedic fascination with grammar and the divine power of speech continued in the tantras in various ways. The most sacred and universal of all Vedic symbols – indeed the most universal of all religious symbols anywhere in the world – the sacred Om, became the foundation of the vast science of mantras in the tantric tradition; and it became the basis of nada-yoga, the yoga of sound.
This process of reinterpretation was carried into detail. Every Vedic mantra has a presiding deity, the sage who revealed it, and the poetical metre in which it is expressed. The tantras used this to give a symbolic – as opposed to a literal, historical – interpretation of mantras.
For instance, the mantra used in tantric worship for purifying the seat or asana says that Meru-prishtha is the sage who revealed the mantra, sutala is the metre in which it is written, and Koorma is the presiding deity. Well, from a literal standpoint, that’s absurd: the sage was not Meru-prishtha, the metre is not sutala, and that makes us doubt whether the presiding deity is indeed Kurma. But symbolically it is deeply meaningful. ‘Meru Prishtha’ means ‘he whose back is (steady, unmoving, and strong as) Mount Meru’, the central axis of the universe. ‘Sutala’ means ‘the foundation of a large building’. And Kurma is the tortoise incarnation of Vishnu who supports the world on his back. So, all three are connected to the idea of a steady, immoveable seat. In this and other ways, the tantras gave a new, spiritually transparent interpretation to a Vedic symbolism which had become obscure and therefore devoid of the power to inspire later Hindu life and culture.
Source - excerpt from an article titled 'The Ramakrishna Movement and Tantra' by Swami Atmarupananda published in the Prabuddha Bharata January 2016 issue page number 51 – 52.