In order to spread Buddhism in a country, an understanding of the conditions of the teaching that had been spread there previously is important.

It is the same as giving medicine to a sick person. It is necessary to know the type of medicine a person has already taken otherwise mixing the medicines may have ill effects or even kill the patient. The same is true with Buddhism. Teachings of the Buddha may also conflict with one another, leaving the people at a loss.

Nanjō Hyōe Shichirō-dono Gosho, A Letter to Lord Nanjō Hyōe Shichirō, Writings of Nichiren Shōnin, Doctrine 3, Pages 143.