So there's two cities he lived in: Mecca and Medinah (Makkah and Madinah). Mecca was almost universally idol worshippers/pagans (not a judgmental word, just one often used in the field). They ostensibly believes in one all powerful God but had numerous intermediaries (idols) which acted as intercessors. In reality, the supreme God was only invoked in times of dire need and for all practical purposes they were polytheists. At the same time, there was little in terms of "theology", at least as we would think about it. There really wasn't a unified creed. People just asked their idol for help and sacrificed to it. They didn't believe in an afterlife or being taken to account for sins.

Medina on the other hand was comprised of two religious groups. One was pagan just like in Mecca. The other was Jewish. Now how Jews got there and why is a different topic (and still not very clear) but they had a strong presence in the city. They were comprised of three tribes and it is probable that at least one tribe followed a version of Judaism that doesn't exist anymore (or perhaps even developed in Arabia).

Now outside of these cities were Christians and Zoroastrians who would have had contact with at least Mecca (it was a trade city). However neither group had a presence in either city.

I am not a historian, so I don't know as much about this topic as an expert in religion or Middle Eastern history might, so its very possible that half of what I'm about to say is based on poor evidence or misunderstandings on my part.

If anyone would like to correct me, go ahead.

There were Christians in the Middle East during the time of Mohammad. The Orthodox Christian Byzantine Empire had managed to convert many of the people in their empire that were living in modern-day Syria, Palestine/Israel, Lebanon, Egypt and Turkey. Many of those places already had large Christian populations, too. There were also Christians in Yemen, Oman, and Ethiopia.

There were also Nestorian Christians in Persia and parts of the Middle East. This church was mainly centered in modern day Syria, Iran and Iraq, but spread as far as China.

Zoroastrians existed in Persia, too, under the Sassanid Dynasty, and was the empire's primary religion until around 650 A.D., after Mohammad died.

There were also communities of Jews living in major cities and towns in both Europe, North Africa, and the Middle East, and there were even Jewish tribes near Mecca and Medina at around the time of Mohammad, such as the Qaynuqa and Nadir tribes, which Mohammad had conflict with in the early 7th century.

However, most of the people surrounding Mecca and Medina around the time of Mohammad were pagans who worshiped numerous gods. The Kaaba in Mecca was actually once a pagan shrine, said to hold 360 idols depicting pagan gods.

Orthodox and other Christianity in the Middle East around 600 C.E. (see section 4.5 and the handy map they have in the top right corner): http://en.wikipedia.org/wiki/Christianity_in_the_7th_century#Middle_East

Nestorian Christianity in the Middle East and Persia: http://www.britannica.com/EBchecked/topic/409819/Nestorian

Zoroastrianism in the Middle East and Persia: http://www.typesofreligion.com/zoroastrianism.html

Judaism in Arabia, specifically relations between Islam and a few Jewish tribes around the time of Mohammad: http://www.answering-islam.org/Authors/Arlandson/qurayza_jews.htm

The birth of Islam, including information on religion in the Arabian Peninsula before Mohammad. This website also has a beautiful map of the religions of major cities in Arabia: http://explorethemed.com/mohammed.asp