Is become a stream-enterer (sotāpanna) enough?

The experience of Nibbana is an unique situation. After the first experience, the person will continue to have that unique experience time to time. Experiencing Nibbana starts the process of involuntary and natural process of purification of this person's mind. It's generally not a very fast process. But after the first experience of Nibbana there is no turning back. The person is guaranteed to become completely free from suffering and attain Arahanthood in the future. It's like being sucked to a black hole. Even you want to turn back, you can't do it. So yes, become a stream-enterer is enough. Either in Seven Life times or in a shorter period of time, Arahanthood is guaranteed. But that max. Seven Life times can be in Heaven Realms too.

So effort is obviously not necessary for a Stream-Enterer to make progress in freedom from suffering because s/he is in an involuntary and spontaneous process of purification and disidentification. Also effort and "trying mode" can be harmful for a Stream-Enterer because s/he is in the transition period. A Stream-Enterer already involuntarily and spontaneously meditates every moment in his/her daily life. But after the transition from Sotapanna to Sakadagami some special methods and returning to the "formal meditator" mode can be helpful to move forward more quickly. But I think we can't call it "effort" because in the next Enlightenment Stages the person has continous peace of mind and contentment. So whatever s/he is doing, we can not say that it is "trying" or "putting effort".

Nyom Damith,

7 more lifes mean 7 times aging, sickness and death: huge suffering not only for one self, but for many, as life requires food.

Does one like to cause another mother and so many others so much pain like in this existence already did? Something one should think about when up to laziness.

My person guesses only Mohayana-confused continue to cause and nurish suffering in the world, telling they make it out of compassion.

Or the livingroom Āhārahats, open to lead a holly life but prefer to be "enlightened householder" with a "secure" refrigerator next.

Things one should really honest consider, especially the "compassionate" ones.

Because people tend to over-estimate there qualities and not seeing that certain easy, merits, are not for sure, the Buddha did not taught much about it, only in one case.

[This answer is given for liberation, not for exchange, stackes, trade... in the world and might be well deleted if space here is not given for such]

Does it mean that a stream-enterer will become an Arahant within his/her next seven existences even without any effort?

Yes.

why would someone put an extra effort to become an Arahant after becoming a stream-enterer?

Because being a stream-enterer still sucks, even compared to being a once returner. A stream enterer hasn't learned how to stop making an effort, so must make an effort in some direction, and seeing which direction will lead to furthur liberation, sustaining a choice for any other direction would take a cumulatively increasing amount of effort.

It's a bit like a dam with a hole in it. The water is going to destroy the dam no matter what. Your choices are:

1. let it happen in its own time.
2. make the hole worse, accelerating the process.

Why would anyone want to be reborn 7 more times?

Buddha enlightens Nibbāna as Arahanta by meditating Noble Eightfold Path, included the right effort in Dhammacakkappavattanasutta:

Precisely this Noble Eightfold Path: right view, right resolve, right speech, right action, right livelihood, right effort, right mindfulness, right concentration.

So, the right effort to cease seven high-level fetters still required for the stream-enterer.

However, the stream-enterer already has the automatic effort of ceasing the three low-level fetters, it guarantees the stream-enterer going to enlighten as Arahanta inside seven rebirths.

Attaining Stream Entry is like attaining a black belt in martial arts (real deal black belt, not McDonald-style handout type). Once one's practiced for that long and that consistent to be able to earn it, any subsequent task s/he has to do after that is no longer a struggling tedious chore. It's become his passion, his way of life. Stream Entry is also like that. No doubt there's still work to be done and effort to be made, but unlike us wordlings who still have to struggle to march forward, the Stream Enterer has done it long enough and hard enough such that the practice has become second nature to him. Hence the natural non-retrogression and the at-most 7 lives warranty.

The Pali word translated as "existence" is "bhava". "Bhava" is a mental asava (defilement) and not another life. There are 10 fetters to break through for Arahantship. The stream-enterer has broken three fetters. Each fetter is a type of becoming. Therefore, the sutta says "they do not take up an eighth becoming" because there are only a maximum of 7 more fetters to break..

In summary, it is pointless asking about stream-entry if you believe in reincarnation because this just perpetuates the "self-view" that must be broken for stream-entry. When there is the belief in reincarnation, it is the belief of "I will reincarnated or reborn seven more times". This is self-view.

The stream-enterer, in the suttas, has the realisation of: "All that is subject to arising is subject to cessation". The realisation of the stream-enterer is not: "All that is subject to cessation is subject to re-arising".