I would start off by saying that the Apostle's travel to the Malabar coast and its impact on early Christianity in the subcontinent is a topic of debate.1 This is mainly because there is not much record beyond the legend about the journeys of the man. From the limited sources that are available two likely routes of entry are speculated. One is the most widely accepted view that St. Thomas came through the sea route into the Malabar. This is further validated by the presence of St. Thomas Christians in the region to the present day. The other sources indicate Thomas being in Persia, gandhara and western India.2 Given the trade networks of the time both these routes are well within the realm of possibility. In the first century it would have been possible to take a ship from either the red sea or the Persian gulf and move along the coast till the monsoon winds would carry the sails to the coast of Malabar. In the north the caravan routes crisscrossed what at the time would have been the Indo - Parthian empire into the territory of the indo Scythians (Saka empire). This was part of the Silk Route roads. This is considering that St. Thomas himself traveled to India. There is much debate on this account with the counter position that its most likely that only the preaching of Thomas traveled with traders(who at the time were a significant flow of people between the eastern edge of Rome and India). Some scholars suggest that Thomas being a common name in the Levant at the time the legend could be confusing the historical figure with others like Thomas of Cana. This would push the date to the 6th century CE. Either ways considering your question posits the date at 50 CE i'll try and explain the makeup of the subcontinent at that time.

The predominant religion throughout the subcontinent(and central asia) at the time would have been Buddhism. Different sects of Hinduism and Jainism also existed simultaneously and many empires had a mix of religions, cults and sects in their domain. The subcontinent was seeing major political changes at the time. The Mauryan empire had fallen a century ago with the Sunga empire replacing it. This in turn led to the creation of several successor states and also let the Khyber pass open for invasions from the west. So in the 1st century CE the Indus valley, the Gangetic plain and the Deccan were divided among several (often competing) empires. To the west as I mentioned before the Indo Parthians ruled most of Persia and the Indus delta. Some sources mention a king Gudnaphar which some scholars attribute to Gondophares I of Parthia. The Indo Scythians were east of them with the western satraps controlling the Indus delta and the Gujarat coast. The Deccan and central India was predominantly controlled by the Shatavahanas and in 50 CE Satakarni II would have held the throne. One location that is associated with Thomas is Killiana or Kalyan which would have been a major center under the Shatavahanas. To the east the northern coromadel and the coast of present day Orrisa as well as the adjoining inland plains would have been under the Meghavahanas of Kalinga. The coast of present day Bengal would have been controlled by the Vanga kingdom. Up river from Bengal the Gangetic plain was subdivided into numerous small fiefdoms. The Fall of the Mauryas had left the core regions of their control divided with several small successor states. In the next century this lack of a large power in the region would allow the Kushans of central asia to dominate the stretch from the hindukush to present day bengal.

The region south of the Deccan being mostly beyond the ambit of Mauryan control had a very different history. The region in 50 CE would have been split between the Cholas, the Pandyas in the coromandel coast and numerous kingdoms on the coast of Kerala predominantly the Cheras and the Ay kingdom. If we are to follow the view that Thomas the Apostle landed in Kerala in the 1st century, then it is very likely he would have landed in one of the Chera ports. The ports of Muziris(present day kodunkallur) and Tyndis(Tondi) were well connected to the Roman trade networks. Their prominence in the monsoonal trade networks is evident from the mention as a main center in the Roman Peutinger Table. Considering present day Thomas Christians are primarily centered in the region of Kerala with their main holy sites in the hills inland from the location of muziris the likelihood of early Christianity entering through these ports is possible. The later historical figure Thomas of Cana is also said to have built the town of Mahadevapattana near the same port as a christian settlement.

I must say that this is generalized account. The reasons for this is that the 1st century in the subcontinent is marked by the formation of several states after the fall of the Mauryan empire and the Sungas that succeeded them. This means there is a lot of nuance in state formation that I have left out. This is also a very general description of the political history of the time, I’ve not been able to include the histories of several smaller regions. It would be great if more users can discuss further about more specific histories of India at the time.

Other Sources : Upinder Singh - A History of Ancient and Early Medieval India: From the Stone Age to the 12th Century

Romila Thapar - Asoka and the Decline of the Mauryas.

Nobouru Karashima - A Concise History of South India: Issues and Interpretations

R C Majumder - Ancient India

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As you mention, India is a huge country. It has the second largest population in the world today and will probably overtake China within this century. Furthermore, it has 22 official languages and many hundreds of others. Many of the states of India are larger than entire European countries – it is not called the sub-continent for nothing. To make matters more confusing, Pakistan and Bangladesh, while separate countries today, would have traditionally been included in definitions of India and are for the most part modern-day constructions for the world of nation-states. Going into a history of the self-conception of people in this geographical region would be a large undertaking but I would refer you to Romila Thapar’s work, Early India: From Origins to AD 1300 (2003). She is an Indian historian who specialises in Ancient India and was who I was directed to when I first became interested in this topic.

I am going to spend the rest of this piece responding by looking at how Greco-Roman writers conceived of India from roughly 400 BC to the first-century AD. I will be upfront and admit that this can be a problematic approach as it essentially looks at how outsiders have defined the people living in an area rather than how different groups would have conceived of themselves and their own identity but I think it links more to what I think you are asking and also with the Thomas traditions which are an example of contact from the Greco-Roman Eastern Mediterranean world with that of India.

There are two main traditions on how Christianity arrived in India. The mainstream view is the one that advocates for a northerly route through Persia and is largely silent on the extent to which Thomas may or may not have been involved, and the other view is that Thomas took a southerly route, across the ocean, from Judaea to Kerala.

In terms of the former theory, it is entirely possible that Thomas may not actually have gone to India. It is a much less exciting theory for sure, but it is a much safer theory. The Church Historian Eusebius has it that Thomas was sent to Parthia (Historia Ecclesiastica 3.1). He also mentions that Thaddaeus was sent by Thomas to Edessa in eastern Syria (1.12-13 and 2.1). This links Thomas and his activities to a northerly route, probably along the Silk Road, and into India this way.

Eusebius was Bishop of Caesarea Maritima, which is in Judaea, and he probably published his work sometime between AD 313 and 325. Given where he lived and what he wrote about, if it had been a well-known tradition at the time that Thomas went to India then I would have thought that Eusebius would have mentioned it. Despite his flaws he at least tries to discern the extent to which his sources are reliable. Eusebius mentions no tradition and he is also not unaware of India. In book 5, section 10, he talks about one Pantaenus who “travelled as far as India”. Eusebius uses the term Hindous here (more on which later) but does intriguingly refer to the fact that when Pantaenus arrived in India the Gospel of Matthew had gone ahead of him. Pantaenus is thought to have died by around AD 200 and so Christianity is believed to have worked its way into India via this northerly route sometime by the second-century AD.

The general term that Ancient writers from the Greco-Roman world used for India was Hindikē. This is thought to come from the Persian word Hinduš and before that Sindhu, Sanskrit for “river”. It refers to what we today call the Indus river and it is where related terms such as “Hindu” come from as well. The Indus river is in modern-day Pakistan and I think it is fair to say that when Greco-Roman writers are referring to India, they are almost always referring to this north-western area that is the modern country of Pakistan. For example, when Herodotus, writing around 440 BC, refers to the land of Hindikē, he says that while it is the “most populous nation in the known world” (3.94), it more or less ends in the east with a large desert of sand. This desert is probably the Thar Desert that spreads across most of the contemporary border between India and Pakistan.

It is hard to say how far back knowledge of India went in the Greek world. Herodotus is the first to write an inquiry about past events (Historia is Greek for “inquiry”), but it is likely that there was prior knowledge. Herodotus’ knowledge may well come via Persia, whose Empire bordered the Indus region. In fact, Herodotus recounts that India is the 20th province of the Persian Empire (3.94). To be honest, you can find some pretty interesting theories about potential contact between the Greeks and India. I have seen books detailing how the Ancient Israelites were actually lost in the Punjab rather than the Sinai Desert and that Ancient Hinduism indirectly influenced Christianity through contacts with the followers of Dionysius. I read most of these books in India, so I do not know how widely available they are outside of it. I also would also treat them with care as people like to claim very grand ideas as truth when trained historians are generally quite sceptical (and rightly so) of these ideas.

Regardless, even if we start with Herodotus, by the time we get to Jesus in the first-century AD the information and contact between the Greco-Roman world and India had massively increased. The most notable event from the Greek side during these 400 years was Alexander the Great’s expedition. He arrived around 326 BC and took various geographers and chroniclers with him to study the area, but we are still referring to the north-western area that Herodotus was talking about, namely the Indus region. Romila Thapar refers to the exchange of information being pretty asymmetrical, with a flood of ideas and writings about India from the Greeks but nothing found in the contemporary Indian sources (Early India, 159).