Finally, let us try to answer your first question whether or not vast virtue of Lord Śākyamuni Buddha exists in our minds. The Sūtra of Infinite Meaning (Muryōgi-kyō), which is regarded as an introductory teaching to the Lotus Sūtra, explains how we attain Buddhahood:
“Suppose there was a prince just born to a king and his queen. Growing up day by day, month by month, year by year, the new prince has reached the age of seven. Though unable to attend the affairs of state, he is respected by his father’s subjects and befriended by children of great kings. He is loved by his royal parents, who find it most enjoyable to chat with him constantly. Why is this? It is because the prince is still young. My good people, he who upholds this Lotus Sūtra is just like this prince. His father is Buddhas; and his mother, this sūtra. Just as the prince was born to the king and his queen, Buddhas and this sūtra are united in one to give birth to the bodhisattva, upholder of this sūtra. Suppose, having listened to the Lotus Sūtra, he reads a phrase or a verse of it, or reads it once, twice, ten, one hundred, one thousand, ten thousand times, or one trillion times the grains of sand of the Ganges River—unlimited and infinite number of times. Even if he had not yet obtained the ultimate truth, he would be respected by all the four groups of Buddhists (monks, nuns, laymen, and laywomen) and the eight kinds of gods and semi-gods, protectors of Buddhism. Accompanied by great bodhisattvas, he would always be protected and cared for by Buddhas and completely surrounded by their benevolence. It is because he is a beginner on the way to Buddhahood.”
Then, we come across the following passage in the Sūtra of Meditation on the Universal Sage Bodhisattva (Kan Fugen Bosatsu Gyōbō-kyō), considered to be the conclusion of the Lotus Sūtra: “This Mahāyāna Sūtra of the Lotus Flower of the Wonderful Dharma is the treasure house of all the Buddhas and the eyes of all the Buddhas in all the worlds in the universe in the past, present, and future. It is the seed of Buddhahood, giving birth to all the Buddhas in the past, present as well as future. You must put the Mahāyāna teaching into practice lest the seed of Buddhahood be extinguished.” And also: “This Mahāyāna sūtra is the eyes of all the Buddhas. It is by means of this Lotus Sūtra that all the Buddhas reach the stage of having Five Eyes. The Buddha with three bodies (Dharma Body, Reward Body, and Accommodative Body) was born from this sūtra. This sūtra is the great seal of enlightenment impressed upon the sea of Nirvana, from which the Buddha with three pure bodies was born. The triple-bodied Buddha, therefore, is the source of happiness for men and gods alike.”
Kanjin Honzon-shō, A Treatise Revealing the Spiritual Contemplation and the Most Verable One, Writings of Nichiren Shōnin, Doctrine 2, Page 143-144