In writing on this subject, you will need to be aware that there is ample evidence that soccer does not unite people. For example, playing soccer against one another and even co-hosting the World Cup in 2002 has not brought the people of Japan and South Korea noticeably closer to one another. The violence that often erupts in various countries between fans of different teams also shows that soccer does not always bring people together.
If you are going to argue that soccer does bring people together, you need to argue that it gives people something to enjoy and to have in common. In a sense, it makes for a universal language. People from China can be excited about Manchester United in England or about Lionel Messi as he plays for Barcelona in Spain. At the same time, people in Africa can be doing the same thing. By giving people from all over the world something that they can have in common, soccer brings people together.