Need answers please! Last name history?

Words history tells you zero about a persons history, if you want to know your family history then you research the people not their name

Words which names are do not tell you what country nor religion, what they can tell you is the language the person spoke, heard or was influenced by when taking/being given the name........ Spanish was a language ( and still is) which is heard, spoken or influenced many countries of the world, any one of which the name could have "originated" in and those people could have been of any religion

Ashley is right. Here is what the Dictionary of American Family Names ©2013, Oxford University Press says; basically the same thing, but bigger words.

Mercado Name Meaning

Spanish: from mercado ‘market’, topographic name for someone living by a market or metonymic occupational name for a market trader.

I'm pretty well-read, but I had to look up "metonymic". It means:

a figure of speech that consists of the use of the name of one object or concept for that of another to which it is related, or of which it is a part, as “scepter” for “sovereignty,” or “the bottle” for “strong drink,” or “count heads (or noses)” for “count people.”. (Merriam-Webster)

So, a non-metonymic occupational name would be "Merchant", not "Market".

Some Jews were shop keepers, but so were lots of Christians. With three exceptions, there are no exclusively Jewish surnames; with some exceptions there are no exclusively Christian surnames, either. Since they were Spanish, chances of them being Buddhist are slender; they were either Jewish or Christian.

Mercado means "market" in Spanish. People who owned a market, worked in a market, or lived near a market would be referred to as "Juan Mercado", to distinguish him from Juan Herrera (the blacksmith) or Juan Molinero (the miller). Eventually Mercado evolved into a surname that was handed down from parents to children. Not everyone who worked or lived near a market was related, so many unrelated people wound up being called Mercado. Thus, not all Mercados are related today. The name has nothing to do with religion; people of any religion could have been called Mercado.

You don't research by your surname. You'll run down a lot of blind alleys that way. Not all Mercados are related. You need to research YOUR family, not every Mercado. Start with yourself and start tracing backwards through the records.