There are Four Powers, the Power of Faith, the Power of Practice, the Power of the Buddha and the Power of the Law. The first two are cause generated. The latter two are condition generated, eternal and unchanging. All Four Powers, both causes and conditions must be correct. Nichiren writes along these lines:
"Suppose that a person is burning with fever. If he sits down beside a large body of cold water and stays there for a while, his fever will abate, but if he lies down beside a little body of water, he will continue to suffer as before. In the same way, if an icchantika, or person of incorrigible disbelief, who has committed the five cardinal sins and has slandered the Law, should try to cool himself beside the little bodies of water that are the Āgama, Flower Garland, Meditation, and Mahāvairochana sutras, the raging fever caused by his great offenses would never be dispelled. But if he should lie down on the great snowy mountain that is the Lotus Sutra, then the raging fever caused by the five cardinal sins, his slander of the Law, and his incorrigible disbelief would be dispelled instantly.
Therefore, ignorant people should by all means have faith in the Lotus Sutra. For although one may think that all the titles of the sutras are the same in effect and that it is as easy to chant one as another, in fact the merit acquired even by an ignorant person who chants the title of the Lotus Sutra is as far superior to that acquired by a wise person who chants some other title as heaven is to earth.
To illustrate, even a person with great strength cannot break a strong rope with his bare hands. But if one has a little knife, then even a person of meager strength can sever the rope with ease. Even a person with great strength cannot cut through a piece of hard stone with a dull sword. But if one has a sharp sword, then even a person of meager strength can cut the stone in two." -- Repaying Debts of Gratitude
"To illustrate, suppose that a person is standing at the foot of a tall embankment and is unable to ascend. And suppose that there is someone on top of the embankment who lowers a rope and says, “If you take hold of this rope, I will pull you up to the top of the embankment.” If the person at the bottom begins to doubt that the other has the strength to pull him up, or wonders if the rope is not too weak and therefore refuses to put forth his hand and grasp it, then how is he ever to get to the top of the embankment? But if he follows the instructions, puts out his hand, and takes hold of the rope, then he can climb up.
If one doubts the strength of the Buddha when he says, “I am the only person who can rescue and protect others”; if one is suspicious of the rope held out by the Lotus Sutra when its teachings declare that one can “gain entrance through faith alone”; if one fails to chant the Mystic Law which guarantees that “such a person assuredly and without doubt [will attain the Buddha way],” then the Buddha’s power cannot reach one, and it will be impossible to scale the embankment of enlightenment.
Lack of faith is the basic failing that causes a person to fall into hell. Therefore, the sutra states, “If with regard to this sutra one should harbor doubt and fail to believe, one will fall at once into the evil paths." -- Embracing the Lotus Sutra
Some say that nothing is ceded from the Buddha and the Law (other powers), faith and strong practice alone is sufficient to attain Supreme and Perfect Enlightenment and others say that practice alone is sufficient (Zazen, for example).
To disabuse them of their mistaken views, one need only cite the Buddhist principles of dependent origination and Ichinen Sanzen which posits the unity of the Three Realms: The realm of the five components, the realm of living beings, and the realm of the environment.