The Soka Gakkai rationalizes their interfaith practices by the following two passages from the writings of Nichiren:
"People can attain enlightenment in two ways: by meeting the Buddha and hearing the Lotus Sutra, or by believing in the sutra even though they do not meet the Buddha. Even before the advent of the Buddha, some Brahmans in India realized the correct view of life through the four Vedas. In China before the arrival of Buddhism, some realized the correct view through Taoism and Confucianism. Many bodhisattvas and ordinary people, endowed with keen faculties, perceived [even before they heard the Lotus Sutra] that Shakyamuni had planted the seeds of Buddhahood within them in the days of the Buddha Great Universal Wisdom Excellence or in the far more distant past [when he attained his original enlightenment]. They understood this by hearing the Mahayana sutras of the Flower Garland, Correct and Equal, and Wisdom periods. They were like the pratyekabuddhas [who could perceive the impermanence of life] at the sight of scattering blossoms or falling leaves. These, then, are the type of people who gained the way through teachings other than the Lotus Sutra.." -- The True Object of Worship
"When the Yin dynasty became corrupt and the people were suffering, T’ai-kung Wang appeared in the world and beheaded King Chou of the Yin, bringing an end to the people’s misery. When the Second Emperor of the Ch’in dynasty caused the people to taste bitterness, Chang Liang appeared and restored order to the world, enabling them to know sweetness. Though these men lived before the introduction of Buddhism, they helped the people as emissaries of Shakyamuni Buddha, the lord of teachings. And though the adherents of the non-Buddhist scriptures were unaware of it, the wisdom of such men contained at heart the wisdom of Buddhism." -- The Kalpa of Decrease
Please note that in both these passages, Nichiren is referring to eras before the Latter Day. Referring to the Latter Day, the age in which we live, Nichiren teaches:
"DURING the two thousand years of the Former and Middle Days of the Law, those who embraced Hinayana or provisional Mahayana Buddhism as the basis of their faith and practiced these teachings in earnest could generally obtain the benefit of enlightenment. However, though they believed that this benefit had come directly from the sutras on which they had chosen to rely, in light of the Lotus Sutra, no benefit ever originated from any such provisional teachings. The reason [they were able to attain enlightenment] is that all these people had already established a bond with the Lotus Sutra during the lifetime of the Buddha, though the results they gained varied according to whether or not their receptivity had fully matured. Those whose capacity to understand the Lotus Sutra was fully mature attained enlightenment during the lifetime of the Buddha, while those whose capacity was inferior and immature [could not attain enlightenment at that time. But they] reappeared in the Former Day of the of the Law, and by embracing provisional Mahayana teachings such as the Vimalakīrti, Brahmā Excellent Thought, Meditation, Benevolent Kings, and Wisdom sutras, they gained the same proof of enlightenment as that obtained by those of higher capacity during the Buddha’s lifetime.
Thus the Former Day of the Law possessed all three: teaching, practice, and proof, whereas in the Middle Day of the Law, there were teaching and practice but no longer any proof. Now in the Latter Day of the Law, only the teaching remains; there is neither practice nor proof. There is no longer a single person who has formed a relationship with Shakyamuni Buddha. Those who possessed the capacity to gain enlightenment through either the provisional or true Mahayana sutras have long since disappeared. In this impure and evil age, Nam-myoho-renge-kyo of the “Life Span” chapter, the heart of the essential teaching, should be planted as the seeds of Buddhahood for the first time in the hearts of all those who commit the five cardinal sins and slander the correct teaching. This is what is indicated in the “Life Span” chapter where it states, “I will leave this good medicine here. You should take it and not worry that it will not cure you.”
In another writing, The Daimoku of the Lotus Sutra, Nichiren again is absolutely clear:
"Now the two thousand years of the Former and Middle Days of the Law have passed, and we have entered the Latter Day of the Law. In such an age, it is a hundred, thousand, ten thousand, million times more difficult for ordinary people to attain Buddhahood or rebirth in the pure land than it was for even the persons of the two vehicles or icchantikas who lived when the Buddha was alive. And yet people nowadays think that, by relying on the Meditation Sutra or some other of the sutras preached in the more than forty years before the Lotus Sutra, they can escape the sufferings of birth and death. How futile, how utterly futile! Women, whether they live at the time of the Buddha or in the Former, Middle, or Latter Day of the Law, cannot attain Buddhahood through any teaching but the Lotus Sutra. None of the other sutras expounded by any of the Buddhas anywhere can help them. The Great Teacher Teacher T’ien-t’ai Chih-che, who heard the Buddha’s teachings at Eagle Peak and later attained an awakening in the place of meditation, has stated unequivocally, “The other sutras only predict Buddhahood . . . for men, but not for women; . . . This sutra predicts Buddhahood for all.”
In the Opening of the Eyes we read:
"The Great Teacher Dengyō declares: “The Former and Middle Days are almost over, and the Latter Day is near at hand. Now indeed is the time when the one vehicle of the Lotus Sutra will prove how perfectly it fits the capacities of all people. How do we know this is true? Because the ‘Peaceful Practices’ chapter of the Lotus Sutra states, ‘In the latter age hereafter, when the Law is about to perish, [accept and embrace theLotus Sutra]....’”
"And Eshin says, “Throughout Japan, all people share the same capacity to attain Buddhahood through the perfect teaching.”
and lastly, further down:
"The goods and evils of non-Buddhist creeds, when compared with the Hinayana sutras, all represent a bad way. Similarly, the good ways of Hinayana teachings, and the four flavors and three teachings as well, when compared with the Lotus Sutra, are all mistaken and bad. The Lotus Sutra alone is correct and good. The perfect teaching of the sutras preached before the Lotus Sutra is so called from the relative viewpoint; from the absolute viewpoint, it must still be counted as bad. Fundamentally it falls into the category of the three teachings, and therefore it is bad in that sense as well. To practice the highest principles of the pre-Lotus Sutra teachings according to the sutras is still bad."
Could we possibly assert, as the Soka Gakkai, that the principles of Islam, Christianity, and Brahmanism, as taught and practiced today, are the highest principles of the pre-Lotus Sutra teachings, let alone equal to the Lotus Sutra as they assert?:
What the Soka Gakkai practices is not the Lotus Sutra Buddhism of Nichiren Daishonon. I can site dozens of additional passages of proof if you are not yet convinced.