It is preached in the Lotus Sūtra, chapter 15 on “The Appearance of Bodhisattvas from Underground”:

“There are bodhisattvas as many as the sand of 60,000 Ganges Rivers originally in this Sahā World, and each of them is accompanied by followers also numbering 60,000 times as many as the sands of the Ganges River. They will uphold, read, recite and expound this sūtra.”

When the Buddha had said these words, the earth of the one billion countries of the Sahā World all trembled and split open and out of them emerged simultaneously immeasurable thousands, ten thousands, millions of bodhisattvas. …

These bodhisattvas had four leaders. The first was Superior Practice, the second was Limitless Practice, the third was Pure Practice and the fourth was Steadily Established Practice. These four were the foremost leaders and guiding teachers of all in the group.”

Grand Master T’ien-t’ai interprets this scriptural statement in his Words and Phrases of the Lotus Sūtra, “These bodhisattvas are My (the Buddha’s) disciples, who will spread My dharma;” and Grand Master Miao-lê comments on this in the Annotations on the Words and Phrases of the Lotus Sūtra, “Śākyamuni Buddha is the father and bodhisattvas who emerged from underground are His children, so the children are going to propagate their father’s dharma;” while Tao-hsien explains this in his Supplement to the Annotations on the Words and Phrases of the Lotus Sūtra, “Speaking of entrusting the dharma, this sūtra was entrusted only to the bodhisattvas who emerged from the earth. Why was this? Because it was the dharma attained by the Buddha in the eternal past, it was entrusted to those bodhisattvas guided by the Buddha in the eternal past.”

For these great bodhisattvas to deliver benefits to the people in the Latter Age of Degeneration is as easy as fish swimming in the water and birds flying in the sky. For those who were born in the evil world to meet these great bodhisattvas and have the seed of Buddhahood sown in them is like the spirit of water that faces the moon and pours out water or a peahen that becomes pregnant upon hearing the sounds of thunder. As T’ien-t’ai says of this in his Profound Meaning of the Lotus Sutra, “As all rivers pour into the sea, bodhisattvas are born drawn by karmic relations.”

Soya Nyūdō-dono-gari Gosho, A Letter to Lay Priest Lord Soya, Writings of Nichiren Shōnin, Doctrine 3, Pages 162.