Is Enlightenment a momentum with a continuum or can have regressions?

Good householder

(if wishing for the Bodhisatta path, don't read it, as good householder could gain liberating wisdom by it. Read only if wishing to abound doubts!)

As for the awakening that arises by practicing the Noble Eightfold Path, once one has gained the first stage of awakening, streamenter, there is actually no more return but one is headed toward liberation within max. 7 further "pleasing" existances in the human- or Deva-realm. Time can be of course shortened or, if no right effort is made paths and fruits arises by birth, by death.

As for other sects, outside the Buddha-Sasana, which desire long wandering on and suffering, because of the fact that once right view is gained, the path cause developed, they but all effort into avoid insight and wisdom as well as to [listen to the good teachings](Listening to the True Dhamma) since if they would hear the Dhamma of the Awakened with proper attention, they would lose their possibility to "compassionate" give and experiance suffering for long periods.

There is, how ever, a point of return (44-49. Turning Away Through Perception, etc. (saññā-vivaṭṭādi)) when touched and in progress with gain of seeing, Vipassana (when having seen the reality of Dukkha), like when listening to the good teaching, when one would need to let go of the last distruction to cling on to hold a stand.

But even if one has gained the stage of Faith-follower or Dhamma-folower, once the Ariya-Dhamma is known, one would die before not having gained to be headed for liberation, since latest by upcoming of death the decay of clinging to a stand is not avoidable and adds the practice to the already known.

The primar cause of staying in Samsara is to associate with fools and avoid wise and awakened people, seeking for giving toward the lower and maximum equal and avoid increase of relation with more advanced and Sublime in regard of virtues, concentration and wisdom.

So one has, if gained good human birth, not done one of the 6 grave transgression, not a fool and touched by the Juwels (e.g. living near where they might still exist) always the choice whether give into the momentum of old-age, sickness and death or toward Unbound.

Less are capable to change their Ancestorship.

These days there are some who wish for Buddhahood, and wisdom-oriented Buddhahood at that, though their conduct barely qualifies them to become ordinary disciples. What characterizes them is the bold banner of craving-de- pendent deeds, which cry out for public recognition right now and yearn for glorious results hereafter....The Four Special Characteristics of a Bodhisatta

Gati, gati, paragati, parasamgati, bodhisvaha

[Note that this is not given for stacks, exchange or any other trade binding to the world (Bodhisatta-way) but for Unbound, escape from Suffering by abounding foolishness]

I don't believe it is an of/off state. Enlightenment is just that moment of understanding your place in the universe. It doesn't stop you doing stupid things afterwards. You are only human and open to the same temptations that you were before.

This answer is from the Theravada perspective.

Stream entry is the point at which the teachings become fully understood, not just intellectually, but also through one's experience.

When a stream entrant feels jealousy due to deeply ingrained latent tendencies and habits, he can quickly see how his jealousy relates to his clinging and craving. He is not free from suffering yet, but he can see how everything works. He also has had a glimpse of Nibbana, at least in moments of mindfulness (sati sampajanna).

The stream entrant has completely uprooted the first three fetters - doubt on the Buddha's teachings, attachment to rituals and rules, and belief in a self (or self-view). He has lost his self-belief or self-view, but not his self-habit. There are no more doubts because the teachings have been fully understood - it's very clear what everything means and how everything fits together.

In SN 13.1, the Buddha taught that the suffering remaining for a stream entrant is like dirt at the tip of a finger nail, compared to suffering that has been endured, the size of planet Earth.

So, stream entry is that point of no return when one cannot go back to complete ignorance of the four noble truths, the noble eightfold path, the three marks of existence and dependent origination.

Here's an analogy. Stream entry is like that point in time where you have completely understood how diet, exercise, obesity, metabolism, homeostasis, ageing and non-communicable diseases work. There's no more denial, delusion and doubt. There's no more ritualistic practice of diet and exercise without understanding. The remaining journey to attainment of Arahatship, is like the rigorous journey of improving one's diet and exercise further till perfect health and fitness is achieved.

Do you see an old woman or a young one? Or can you see both?

Once you've seen, can you forget how to see it?

Until we see the world as it is we don't see it, but once we've seen it we cannot 'unsee'. We may fall back on old habits and patterns (old ways of seeing the world), but enlightenment (honestly) is just seeing the world as it is. It's our natural state, something we're drawn away from by our egoic selves, but which pulls us back to itself whenever we stop fighting against it.

I know, sometimes people feel the need to walk all the way around the block to get back to their own front door; the craving for progress can delude us as easily as any other craving. C'est la vie.