While you've already received some nice suggestions I'm quite surprised no one mentioned Jerusalem .
Jerusalem is considered holy to the three major Abrahamic religions—Judaism, Christianity and Islam.
The old city is divided into four quarters—known since the early 19th century as the Armenian, Christian, Jewish, and Muslim Quarters and religion is a very big deal there and one of the most impressive city I've ever visited.
The old city being smaller than the cities mentioned by others and packed with so many different religion
I feel it's a better candidate than most of those cosmopolitan cities.
travel.stackexchange.com/users/2523/Geeo
A close proxy for "most religions" that is much more easily measured will be what is called a "Global City," and Wikipedia happens to have exactly such a list .
The criteria used in determining their ranking are: A variety of international financial services, notably in finance, insurance, real estate, banking, accountancy, and marketing Headquarters of several multinational corporations
The existence of financial headquarters, a stock exchange and major financial institutions Domination of the trade and economy of a large surrounding area Major manufacturing centres with port and container facilities Considerable decision-making power on a daily basis and at a global level Centres of new ideas and innovation in business, economics, culture and politics Centres of media and communications for global networks Dominance of the national region with great international significance
High percentage of residents employed in the services sector and information sector High-quality educational institutions, including renowned universities, international student
attendance[8] and research facilities Multi-functional infrastructure offering some of the best legal, medical and entertainment facilities in the country Now clearly not all of these criteria will directly correlate to cultural (and therefore religious) diversity, but it's a close proxy.
And rounding out the top of their list, with Alpha++ rating are: London, UK New York City, USA
The cities with Alpha+ rating are: Hong Kong Paris, France Singapore Shanghai, China Tokyo, Japan Beijing, China Sydney, Australia Dubai, United Arab Emirates
Now, you'll want to apply your own filter to this list, as various religions have different legal statuses in some of these places.
But this list should be a good place to start looking for culturally-diverse cities you might want to visit.
travel.stackexchange.com/users/444/Flimzy
Go to London.
In the metropolitan area (covered well by cheap public transport) you can find many, many different religions.
Christianity (many different branches of), Judaism, Islam, Buddhism, Sikhism, Hidus, etc, etc, etc.
Also, any big metropolis will be the same.
http://en.wikipedia.org/wiki/Religion_in_London
travel.stackexchange.com/users/663/Darko Z
With regard to the most religiously diverse country and your goal of seeing the most religions in a short time, probably Singapore is your best bet.
It likely has the most distinct and heavily-practiced religions per km², amongst all the countries of the world (though probably not amongst all the cities of the world).
travel.stackexchange.com/users/8803/Kenny LJ
No India on the list.
That's bull.
I live in New Delhi , the capital of India and believe you me every part of it is diverse.
India as a country is diverse.
You will find Hindus, Muslims, Sikhs, Jews, Christians, Buddhists, Jains.
Tibet's government in exile, the administrative operations are from India.
The dalai lama temple is in India.
There are loads of other examples I can give.
In short, the most religiously diverse country in the world is INDIA.
You will find A Church, A Temple , A Mosque, A Gurdwara in every city of the country.
Not to forget, INDIA is the birthplace of 4 major religions : Hinduism, Buddhism, Jainism and Sikhism.
Apart from this, we have roughly 20% of population to be Muslims.
Also, roughly 5% of total population are Christians and let me stress that the North Eastern States of India namely Mizoram and Manipur have more than 90% of population to be christians.
travel.stackexchange.com/users/8617/madLokesh
I really like to learn more about religions, and even though I'm learning some information from the internet, I would like to learn from real experience.  So basically I'm searching for a country that is composed of people of different religions; the more religions, the better.With different sects of each religion.Also, the country should have a small area (i.e I can go to several groups/sects in little time.)  Is there any way to determine the most religiously diverse city or country?
Which city has the greatest religious diversity?
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<culture><countries><cities><religion>
travel.stackexchange.com/questions/25601
travel.stackexchange.com/users/12717/Mat
India is the most religiously diverse country in the world and the birthplace of 4 major religions, while Jerusalem is considered holy to the three major Abrahamic religions—Judaism, Christianity and Islam. Another option is a cosmopolitan city like London. A number of countries and cities vie for the position of most religiously diverse, including London, Singapore, Jerusalem and India. 
India has a great many religions and is the birthplace of 4 major religions: Hinduism, Buddhism, Jainism and Sikhism. Jerusalem is considered holy to the three Abrahamic religions: Judaism, Christianity and Islam. London has many diverse cultures and religions Singapore is the most religiously diverse country.