Nichiren may indeed have included the three major forms of Western monotheism if he had known about them, as he seems to have wanted to account for all the major religions in the world. I also believe that he would have evaluated them using the same method of comparison in terms of the scope of time scales. To review: just as the Buddha criticized the sixty-two (or ninety-five) views of his contemporaries who drew dogmatic conclusions about the nature of life based on limited experiences either in this life or even from past-life recall, Nichiren evaluated Confucianism, Taoism, Brahmanism, Buddhism, and other philosophies and religions based on how limited or vast a scope of time their teachings accounted for. Confucianism fares the worst for not even attempting to account for life before birth or after death but limiting itself to teaching morality only in terms of the present lifetime.

Brahmanism fares better for it does teach that there is a cycle of rebirth that unfolds according to the law of karma, and thus accounts for a much greater scope of time. In fact, Brahman cosmology teaches that there are whole cycles wherein world systems are created, maintained, and then destroyed over the course of eons and within those cycles beings are reborn continually until they can attain one of the heavenly realms. The Upanishads taught that those who realized the Ātman or True Self would be forever liberated from these cycles, but the Buddhist sūtras do not mention the Upanishads nor does Nichiren.

From the Buddhist point of view, however, in the course of time even those reborn in the heavens will exhaust their merit and they will have to be reborn elsewhere depending on what causes are able to come into fruition. From the Buddhist perspective even the vast amounts of time spent in a hell-realm or a heavenly-realm is still a finite period of time because all caused and conditioned states will eventually come to an end. Such is the universal law of the impermanence of all conditioned phenomena.

Coming back to Western monotheism, … The important thing is that the mainstream view posits only one lifetime to be followed by an eternal afterlife of some sort. Going by Nichiren’s criteria, I think he would perhaps have placed Western monotheism ahead of the agnostic Chinese schools of thought because it at least provides for some kind of afterlife wherein the causes one makes in this life will come to fruition for good or for ill. On the other hand, I think he would not have put Western monotheism on the level of Brahmanism, as the latter accounts for many lifetimes and its understanding of the unfolding of cause and effect over many lifetimes is more developed. From a Brahmanist point of view, one might live in heaven or hell for thousands or millions of years, but it is not actually an eternity though mistaken as such by those who don’t see larger time scale. I stress, again, that this is my guess based on how Nichiren evaluated the other non-Buddhist traditions.