I've been deciding recently (along with everyone else, really) that every religion is just a slightly different cultural expression of the same basic truths. And I've been reading various Hindu texts recently, too, and now I have a nice little comparison chart that basically says that Hinduism and the Near Eastern Monotheisms (Judaism, Christianity, Islam) follow the same developmental cycle. It goes azoy:
Way back when, you have a group of people who worship some sort of god(s) who really like(s) sacrifices. This is the Vedic period of Hinduism, and it's ancient Judaism, along with all the other ancient religions of the Near East and even Greek and Roman religions. Judaism seems all special due to the whole monotheism thing, but Paula Fredrikson argues that everyone, even the Jews, was really henotheistic back then (basically only worshipping one main God, but believing in all these other lesser gods- the Jews called them demons, but they were exactly the same as the gods of their neighbors), which is exactly what a lot of Vedic tradition is- one hymn is dedicated to Agni, another to Surya, etc, and each one claims that it's chosen god is the supreme being. And it's not really that every Hindu believes that every god is supreme at some point, but more that various groups of brahmins focus on the sacrificial rites for various gods- not so different from the ancient polytheisms of the Near East and Mediterranean, eh? In Hinduism the years on this are about 2000-1100 BCE. For what I'm going to call the "Western religions," these things are definitely happening around then, but it really continues in that vein until at least 200 BCE and probably more until like 200 or 300 CE. (Hinduism remains about 400 or 500 years ahead of the Western religions for the rest of this development.)
So then, around 800 BCE, a bunch of charismatic Hindu sages (some brahmins, i.e. the traditional priests, but some not) start moving away from the cities and into the forest and starting ashrams, or communities of disciples, where they start teaching a more esoteric/mystical version of Vedic philosophy. Some of these ashrams get really radical and start totally new religions (Buddhism is the most prominent of these), but many of them continue to affirm the Vedic tradition, they just reinterpret it. These people compose the Upanishads, which speak about an ability to develop a more direct relationship with the divine, regardless of one's caste (or gender). Many of these sages are ascetics, and this is the time that various yoga practices become central.
Now, in the Near East, around the first century BCE or so, and continuing until the sixth century CE, all sorts of people, some traditional priests, but many not, start becoming charismatic leaders and reinterpreting their religious traditions. The ones you've probably heard of include people like Plato and Jesus, who of course got all sorts of disciples and each ended up founding new religions based on radical new interpretations of their respective traditions. And their followers started moving into the desert and getting disciples of their own and practicing ascetic lifestyles and renouncing the world to achieve spiritual fulfillment.
So now we have two or three religious traditions moving from sacrifical practice done by a special priestly class to a mystical vision of the world where ascetic practice allows any person (although only those who can renounce the world) to experience the divine.
And then, between 200 BCE and 200 CE, people in India start to rethink their religions. Two giant epic poems get written that reflect these new modes of thought. One of them, the Mahabharata, includes a section known as the Bhagavad-Gita, which explains a radical new way of experiencing Hinduism. In it, Krishna, an avatar of the god Vishnu, has a long conversation about the nature of life and the universe and everything with a man named Arjuna. Krishna explains various ways of connecting with the divine, but it ultimately boils down to the idea that anyone who loves God can connect with him.
In the Near East, in the sixth century CE, a man named Muhammad starts talking to an avatar of his God named Gabriel. Gabriel tells Muhammad all sorts of things about the nature of life and the universe, and presents a new interpretation of the religion Jews and Christians have been practicing for a while. This version focuses on things that every person can do within the confines of daily life to become beloved of God. (This is where I need to say that this is in fact different from Christianity- although modern Christian practice is very incorporated into daily life, this is only due to necessity. Jesus is always saying "leave everything and follow me." Early Christians were super anti-marriage and really into monasticism, both of which are really hard for ordinary people to conform to. Islam never wanted people to sell everything and give it to the poor, and it always affirmed things like marriage and having a job. Just as Judaism now has nothing to do with sacrifices, I am looking only at the early manifestations of each of these traditions.)
What does all this mean? Hell if I know. India and the Near East aren't that far from each other, and there is definitely documentation of Hindu Upanishadic practice influencing Near Eastern ascetics. Maybe this is just a natural progression of religions that want to be more relevant to more people over time (so they have to open up). Judaism made the same accomodations within itself that gave rise to Christianity and Islam. Hinduism is different because people who follow the Gita still affirm the entire Vedic tradition- there's more unity and fewer splinter groups. But you have things like Buddhism, which did splinter completely from Vedic Hinduism. One thing that I think I can pull from this is a theory of where other shamanic traditions that ultimately fell by the wayside may have moved- tribal religions of Africa, South America and Europe are very similar to early Vedic Hindusim. The word for an Upanishadic sage is the same Indo-European root as the English word "shaman" ("shramana"). (Indo-European, by the way, is the super-ancient language spoken by the ancestors of pretty much everyone who lives in Europe and India these days- all European languages except Hungarian, Finnish and Basque and pretty much all of the languages of India are related to each other.) Monotheism isn't a given, but people seem to prefer focusing on one god to dealing with a whole lot of them.
The obvious question that comes out of this is- what's the next step? It's been 1200 years since Islam began, and 1800 since the Gita became popular. Religion has changed in that time, but not nearly as much. I'd argue that what we call the Enlightenment and the rise of "secularism" is actually that new religion- scientists are a sort of priest, now. What that means, ultimately, I don't know.