What is Ichinen Sanzen (3000 Worlds in a Moment of Life)?

In the Latter Day it is Namu Myoho renge kyo and Gohonzon. This is Actual Ichinen Sanzen, 3000 Worlds in a Moment of Life is Theoretical Ichinen Sanzen.

Theoretical Ichinen Sanzen:

Three Thousand Realms (or Worlds) In a Momentary Existence of Life 

Each of the Ten Realms [Hell, Hunger, Animality, Anger, Humanity, Rapture, Learning, Self Realization, Bodhisattva, and Buddha] is manifest from moment to moment and contains Ten Realms which are latent. This makes One Hundred Realms. Each of the One Hundred Realm contains Ten Factors (appearance, nature, entity, power, influence, internal cause, relation, latent effect, manifest effect, and their consistency from beginning to end). This makes One Thousand Realms. Each Realm also contains the Three Principles of Individuation [the Realm of the Individual, the Realm of the Society of Beings and the Realm of the Environment. This makes Three Thousand Realms. 

In general, a Bodhisattva [selfless being] manifests primarily Bodhisattva from moment to moment and always quickly returns to Bodhisattva when manifesting one of the other realms. The Bodhisattva has the Appearance, Nature, Entity, Power, Influence, Cause, Condition, Latent Effect, Manifest Effect, and Consistency from Beginning to End of a Bodhisattva. Both the society or collection of beings in which the Bodhisattva resides and the environment [land] of the Bodhisattva manifest predominantly the Realm of Bodhisattva. 

In general, a cat manifests primarily Animality from moment to moment and always quickly returns to Animality when manifesting one of the other realms. The cat has the Appearance, Nature, Entity, Power, Influence, Cause, Condition, Latent Effect, Manifest Effect, and Consistency from Beginning to End of an animal. Both the society or collection of beings in which the cat resides and the environment of the cat manifest predominantly the Realm of Animality. 

A Bodhisattva will manifest Animality for periods of time when, for example, he experiences the urge to sleep or defecate. A cat will manifest Bodhisattva for periods of time when, for example, it acts selflessly towards its kittens or its owner. 

Tientai asserted that every being has the potential to elevate its central life tendency [or the Realm or Realms to which it gravitates] through the principle of Ichinen Sanzen. For example, a cat can create the causes and conditions for Bodhisattva by either very strongly [for a moment] or habitually acting selflessly. Intent, according to Tientai, is far less important than actuality or suchness in elevating [or lowering] one's central life tendency. 

The practice advocated by Tientai was meditation directed towards perceiving the Three Thousand Realms In a Momentary Existence of Life. He recited the title of the Lotus Sutra ten thousand times a day for his personal practice but since the time and the people's capacity were not ripe for the widespread propagation of the principle practice of the Lotus Sutra, he refrained from advocating this practice for others. 

Actual Ichinen Sanzen:

Nichiren Daishonin taught Actual Ichinen Sanzen or one moment of exceptionally profound faith and joy in Namu Myoho renge kyo and its continuous recollection is itself Buddhahood.

More on Ichinen Sanzen:

In This Latter Age, People Principally Cycle through the Lower Six Worlds.

Question: How can that be? Tientai describes Three Thousand Worlds In a Momentary Existence of Life [Ichinen Sanzen]?

Answer: Ichinen Sanzen is a “non-random generator” but appears to the unenlightened as a random generator. From moment to moment, any one of the Three Thousand Worlds may manifest but not randomly. This is akin to the non-random generator of a slot machine which appears to the slot player as random. The slot machine, like Ichinen Sanzen, is set for most pulls [moments] to lead to no return on the money (Hell), little return on the money (Hunger, Animality, Anger, Tranquility, and Rapture), some return on the money (Learning and Self Realization), a small jackpot (Bodhisattva), and the super jackpot (Buddha). We set the non-random generator of Ichinen Sanzen though our thoughts, words, and deeds, just as the engineer sets the non-random generator of the slot machine with his knowledge of physics. The functioning of the non-random generator of our Three Thousand Worlds from moment to moment is determined by the Law of Myoho renge kyo [cause and effect] while the functioning of the non-random generator of the slot machine is determined by the Law of physics.

It is as rare to continuously hit the super jackpot pull after pull as it is to continuously manifest Buddhahood from moment to moment. However, one may continuously hit the super jackpot if one has a powerful properly configured electromagnetic device attached to the slot machine which alters the generator of the machine [making it always come up super jackpot]. In like manner, chanting Namu Myoho renge kyo with a correct faith can alter the generator of Ichinen Sanzen so many or all of our life moments come up Buddhahood. Shakyamuni Buddha of the Juryo Chapter of the Lotus Sutra is an example of one who has manifested Buddhahood since the infinite past. Nichiren Daishonin also strung together many moments of Buddhahood which will carry over into the next life. We too are able to string together many moments of Buddhahood through a correct faith and practice of the Lotus Sutra.