I have told this to Atheists before in paraphrased way:
The only way to justify Atheism, would be, to say they have either proven that clear reasoning doesn't affirm God nor are we connected to him. As for the latter, in reality, they would have to prove God doesn't exist to do that since if God exists, it's irrational to think we are not connected to him. And reasoning that is clear and manifest, doesn't mean it has to be what Academia thinks is the best arguments for God. It can be as simple as we require a perfect vision and witness and judge to be defined as human beings. And that can be sufficient proof (and in fact is, one that Quran repeats).
The problem with taking a stance that others haven't provided enough proofs, which is a stance of denial of evidence, is that it takes constant research and reinvestigation - and one can never be certain they haven't digested the arguments well enough nor can they be certain of there not being arguments out there that do prove what they are saying they have no reason.
Now if they sincerely seek the truth and don't find it, and do their best, than yes, they are justified. But all three are questionable. Are they seeking the truth (what proof do they have?), did they not find it (again, perhaps they did but are ignoring the proofs of that truth), and did their best (what proof, are they constantly looking, are they seeking, and they reinvestigating, making notes, brainstorming etc).
So to relate it with Sunni-Shia issue, I think the stance of lack of evidence of something is not a stance God would want any human to take.
Because really - the Quran which is a guidance to all things needed by humanity - what limit can we put on it in the first place?
Second going down this - it could been more clearer route, is dangerous. What is clear enough should be sufficient because God always sent signs after one another, on bigger than the other, but people didn't believe, they kept raising the bar.
They made a demand "If you show this and that, we will know you are a Messenger", but when shown, kept raising the bar.
They may really rightfully ask why doesn't God just decisively prove it, and that is true, God can, but there is a trial in all this, and there is the reality of faith and disbelief....and honesty and deceiving oneself, and seeing by love of God and seeing by desires towards the false world.
If Quran was repeating "There are no divinely chosen leaders after Mohammad" or something like that, and Sunnis had clear decisive proof for their case, than they would not be hypocritical by demanding proofs whence they themselves have none.
But the truth is Quran has everything pertaining to guidance and there is wisdom in the structure of it, Surah to Surah, verse to verse, theme interlinking with other themes, and verses connecting themes to one another.
The Quran proves Ali and the Leaders appointed by God for guidance to me, without a shadow of a doubt.
Relying on doubt and skepticisim as opposed to clear proofs from God has never been the way to God in the past so as to be the way now.
Sunnism is taking a skeptic stance towards Ghadeer declaration, thaqalain hadith, hadithal Kisaa etc, but at the end, it has no proofs of it's own from God.
It denies proofs that contradict it, I understand that, but it has none for itself. And the path of God always has proofs upon proofs and so much reasoning to confirm it, that all it requires is reflection. The latter statement is found in Quran and itself is enough to prove the path and in fact, is one of the arguments of Quran for the proper religion, that at least Islam hypothetically has evidence, people may say Quran doesn't prove Mohammad but no other religion has evidence that is to be analyzed, they are saying walk the path and you will see, but Quran is saying, the path before you walk it, make sure it has ample evidence and proofs.
And the Quran provides proofs upon proofs, majestic eloquent themes interlocking with others, and in all this, proves the proper religion through reasoning.
The argument of Quran doesn't come down to he sent Messengers in the past therefore believe in Mohammad, as any human can say the same thing. It has structural argument and reasoning and philosophy, and there is no wasted space in Quran, all verses are important.
The philosophy of chosen families, the number emphasized by Moses covenant, the rejection of clergy class and the Jibt and Taghut, the tautologies of Quran, etc, etc, they are all vitally important.
All I suggest before going into all out rebellion to the family of the reminder, is to reflect on Quran and ahadith of Ahlulbayt (as) regarding Quranic verses.
You should at least give Ahlulbayt (as) a chance to prove themselves in Quran by reading their ahadith and giving them reflection.