Christians believe in God and they believe that Jesus was the most important prophet. In the case of Catholics, they perceive the pope as the messenger of god. All Christians believe that believing in Jesus will save them from their sins. The most curious thing is why Christianity has many religious orders. They all believe in God and they see Jesus as Messiah, but they have different specific beliefs. Some branches say that one can only be baptized if they have complete faith in that branch. Others say only children should be baptized, or only adults. Some sects (like the Eastern Orthodox) are strongly opposite to the Roman Catholic beliefs. Subdivisions like the Mormon church abstain from all alcohol, or anything that affects judgment. Many have other beliefs, but those were the main ones to cover.
And then again, why is Christianity so spread out? The answer lies in Roman Emperor Constantine. You see, at that time (312 AD)Rome was a huge empire. When Constantine converted from pagan to Christianity, the rest of Rome did too. So the whole of huge, giant Rome became Christian, which led to the spread of Christianity. On top of this, European explorers went to explore the New World. A few years after they returned to their home countries with many stories of their conquests, the whole of the New World was swarming with priests, aiming to convert the First Nations to Christianity.