Perennial philosophy? Back to Wikipedia:

The Perennial philosophy (Latin: philosophia perennis),[note 1] also referred to as Perennialism and perennial wisdom, is a perspective in modern spirituality which views each of the world's religious traditions as sharing a single, metaphysical truth or origin from which all esoteric and exoteric knowledge and doctrine has grown.

A more popular interpretation argues for universalism, the idea that all religions, underneath seeming differences point to the same Truth. In the early 19th century the Transcendentalists propagated the idea of a metaphysical Truth and universalism, which inspired the Unitarians, who proselytized among Indian elites. Towards the end of the 19th century the Theosophical Society further popularized universalism, not only in the western world, but also in western colonies. In the 20th century universalism was further popularized in the English-speaking world through the neo-Vedanta inspired Traditionalist School, which argues for a metaphysical, single origin of the orthodox religions, and by Aldous Huxley and his book The Perennial Philosophy, which was inspired by neo-Vedanta and the Traditionalist School, culminating in the New Age movement.

I am learning so much in this thread! Thank you all for your participations.

As regards the 'trenchant criticisms', it seems to be a case of the reductionists against the universalists, or to paraphrase something I've seen around the forum, "It's not what you look at, it's what you see that counts."

This certainly deserves more discussion.