What I try to understand is why this has all been done in scriptures, why did God want us to believe in him through eye witnesses and some old written down texts that he knew would spread doubt in peoples mind if people at all would see the scriptures.

Was this done on purpose? Why did he not wait for these miracles to happen when humans had an internet connection and better means to witness it.
Now we have to trust some fishermen to have written down the truth on some single events happening hundreds of years ago. Was this really the only way?
Testing your own creations like this over and over again whilst they would multiply themselves, creating more children that have to spent a lifetime finding out whatthe purpose is of life.

I spoke to a well studied person a while ago in these concepts and he mentioned that if there is a God it would have to be a cruel one, for as most people on earth suffer unimaginably.

We that live in the lucky side of the world just don't feel and see that suffering as much as we live in abundance of wealth.

What if you were born in a country that never heard of those scriptures, brought up by people that can't read, have no internet, nor the time to be scholars of the Bible.
Most people worry about getting through the day.

Why is it that only rich countries have the right to study the bible whereas the poorest countries have no means to read or write and would just be living without the knowledge of this powerful God.
I know the bible mentions something in terms of that if you were unlucky not to see the bible you are still accepted as Gods child.

Life seems a lot easier if you don't get to see the bible according to that sentence for as if you do get to know the bible you must assure yourself that all other belief systems are wrong (it is an almighty task that is humanly impossible) No man can know all, so no man can ever fully conclude that his belief system is the only correct one.
This why it has to be regarded as a belief system that may proven to be correct, but there is a likelyhood that it isn't.

How often do people fool themselves thinking that someone tells the truth and it turns out more information was required to prove it was all a con.
How many people get indoctrinated with different truths all the time, how did Hitler manage to control so many minds.
People follow what is fed to them, based on their environment. This is why someone born in a country that believes that cows are holy will most likely believe that cows are holy. It is based on location, probability and environmental exposure.
For this reason alone we cannot and should not conclude that we can know the truth ever as we stop allowing ourselves to collect more evidence that it isn't.

Why didn't God make it a little bit more obvious to us that his ways are the only ways, but left us all with doubts causing millions and míllions to believe other things.

Is it a sin to believe that all other religions could be true as well unless proven wrong and just accept the fact that we haven't got the time to disprove these religions?

Will the world not be a better place if we allow ourselves to keep all options open?
Like any theory, we have to keep an open mind that new information might come to light, changing everything, in hundreds of years from now people will look at us and laugh about our shortsighted views of the world. As is proven over and over again,

How do we prove something wrong in a world we don't fully understand?

Quantum physics proves us that the world works fundamentally different in terms of physics and it seems that information of particles becomes reality by observing it.

In an information based world where particles can come randomly into our world, how do we disprove other believe systems if the world has shown us that miracles can happen.

Why not leave all possibilities open? Why choose for one truth?

How often do laws of physics only turn out to tell one side of the story but work differently on other levels.

There is no way for humans to be sure what is correct, this is why it is called a believe, I think discarding other options seems shortsighted and assumes that we are capable of knowing the truth.

But there is no evidence that we are ever right? All we do is guess based on information that is fed to us. And there is not enough time in the world to study all that is available or will become available.

The Egyptians lived for hundreds of years believing in a shortsighted view of the world, or maybe they were right? Maybe their reality worked for them? Reality is perception and depending on how you observe it you can create your own believe.