What religion is truly authentic?
Think about it...what religion is truly authentic. Catholics adopted many Pagan symbology and Ideology. From the Saints having "Halo's" to Days of the week, along with the "True" day of Worship....Romans Worshiped Pagan Gods and then in 324 Catholicism became its official religion. But they developed the basis of the religion to self serve they're needs as pagan Sun-god Worshipers. Hence Sun-day being the day of worship.
Actually there is no religion alive today which isn't in some way a reaction to and a continuation of previous religions. For instance:
Buddhism started with Gautama Buddha whose thoughts clearly stand in the tradition of Upanishadic thought. Thoughts on karma and reincarnation and salvation from Samsara (the wheel of rebirth) were clearly being debated at that time. Jina (the founder of Jainism) also spoke on this issue and his solution is clearly different from the Buddha's, but still enough like it that it is clear they stood in the same 'milieu' of thought on this subject.
The Upanishads themselves are the result of philosophic speculation on the meaning of the Vedic rituals. The Vedic rituals can be shown (on linguistic grounds) to be elaborations of previous rituals also shared with the people that moved into Europe as well as with the rituals of Zoroastrianism.
Judaism picked up a lot from the Mesopotamian religions in the Middle East at that time - though it also reacted strongly to some of it (the golden calf: first worshiped, then condemned).
Islam is a late one in this series. It originated in the Middle East at a time when Christianity was already strong there and Judaism hadn't disappeared - it is a clear continuation of the Monotheism in these two, with rituals adapted from both and its version of Biblical events shows that Mohammad probably didn't have access to a Bible, but he clearly did grow up hearing many of the Christian stories.
The further back in time you go, the harder it is to say anything definite. But the above exposition does go back to at the latest 1400 BC (the Rig Veda, a text in Vedic Hinduism, has elements at least that old). The fact that we don't know what was before that, doesn't mean people didn't have religion of some sort.
There is historic evidence that there was a group of people living in (probably) Central Asia who created a set of gods and ritual practices which are the ancestors of the oldest living religions today (Hinduism and Zoroastrianism) - as well as aspects of early European culture. This religion has very little in common with any religion living today - even if elements of it do survive in Vedic and Zoroastrian ritual.
So when is a religion truly authentic?
Does something have to be new or original to be real?
Originality only becomes a problem when exclusivity is claimed. So if Christianity claims to be THE ONLY TRUE RELIGION, it is in trouble if it is shown that many previous religious elements are included in it... (though actually there have been theological explanations of that fact: God reveals His true religion only gradually, you know... - which makes Islam or Bahai better candidates for THE ONLY TRUE RELIGION)
It was thought, in the 19th century, that all religion has to have one ancestor which would be the source of it all and hence the one true religion. Helena Petrovna Blavatsky claimed that this source was the Ancient Mystery Religion - in other words: Source religion = universal religion = occultism = Yoga Vidya = True Theosophy... Historically speaking this is highly unlikely, though Blavatsky's idea is a creative answer to that 19th century assumption.
21st century knowledge does make it clear that the one thing all religion seems to have in common is mysticism. Unfortunately that is also one of the main controversies in organised religion: mystics (and prophets) are always dangerous to organisations. Mystics are the people who breath life into religions, but also the people who are likely to go over the accepted boundaries of religious orthodoxy. The Sufis (the mystics in Islam) in India for instance were very good at incorporating elements from Indian Yoga traditions into their practice. For some of them every tradition wants to claim them as their own. Kabir for instance is seen as a Hindu by Hindus and as a Muslim by Muslims.
Blavatsky would probably say that most mystics are amateur occultists or initiates: initiated by nature instead of by proper adepts.
For me that latter aspect does seem likely, but the historic suggestions Blavatsky makes (including Lemuria and Atlantis) seem highly unlikely... The reason I do take her general theory seriously is that some cultural processes do seem to go from the high to the low - and that is precisely contrary to the direction one would predict based on evolutionary theory. Grammar for instance only became simpler... This is connected to the question this article started with because Blavatsky claims there was once a universal language as well as a universal religion. Language became fragmented (a sort of tower of Babel theory) and true religion became limited to the initiates when ordinary people could no longer be trusted to use the occult powers selflessly.
True Religion is (and can only be) religion that conforms to TRUTH. Everything else is just a collection of thoughts/beliefs and rituals/practices. Hence the theosophical watchword:
There is no religion higher than truth