Roughly speaking, in Japan there are four religions. The major religions of Japan are Shintoism and Buddhism, and 99 percent of the population are Shintoists or Buddhists. The minor religions are Christianity or other sects.
Shintoism is the original Japanese religion. Shintoism is based on an ancient Japanese myth. According to the myth, at the beginning of the world, Japan was born of gods. Many years ago, the Emperor was believed to be the offspring of the gods. This idea was the Japanese driving force of World War II. Buddha in India founded Buddhism. After that, it was brought to Japan by way of China. Anyway most Japanese people have the religions of Shintoism and Buddhism. But not many people go to temples and shrines regularly.
In contrast, the people whose religions are Christianity or other sects are very active. Christianity was brought to Japan in 1549. At present Japanese Christians go to church every Sunday. Unfortunately, some of the sects are very dangerous. For example, several years ago one of the sects committed murder. They made poison gas, and released the gas in the train. Consequently, many people were killed.
In general, however, it is difficult to divide people into religious groups in Japan because most Japanese people go to temples in spring and fall, go to shrines at the first New Year's Day, and also celebrate Christmas.