How do believers know what god thinks and does?
There are two basic ways.
In Romans 1 and 2 we see that everyone is born with an inner knowing that certain things are true: Like God exists, He is your Creator, and you are accountable to do what is right and not what is wrong. Which is why he Bible says all will be judged one day based on this and won't be able to claim ignorance as an excuse for what they did wrong.
More specifically, with regards to Christians, the Bible says it is by the Holy Spirit within them communicating to them through various ways. As well as experiential knowledge developed of God that allows them to, as Jesus said, "know His voice and follow Him".
There is kind of a third indirect way, but it really is just an offshoot of the second way. That is to learn things about God's nature and ways by what other people have had directly revealed to them (ie. Listening to prophets or reading Scripture which is the writings of prophets).
Discerning the validity of the third requires using using the first two. Your ability to discern correctly is going to be based on how much you want truth vs your sin. If you don't really want truth and have a heart clouded by a love of sin you're not going to be able to discern clearly. Kind of like introducing distortion into an old TV or radio broadcast. Introduce enough of it and you won't even be able to pick up anything.
We need to be purified to receive God's truth clearly just as a broadcast signal needs to be purified of an interference in order to come through with clarity.
Even one person says god is compassion and doesn't do harm and the other says he does harm but that means he is just.
It's not harm by definition if it's just.
The only contradiction here is how you interpret God's justice as a bad thing when in reality it is a good and necessary thing.
Executing a serial murderer is not a harm to society but it is justice and healthy for society that such justice is done.
It is quite ironic that most atheists I come across will angrily accuse God of being evil because they think he doesn't intervene to stop evil from happening - but in the same breath will turn around and then angrily accuse God of being evil because he brought judgement on evil people in the Bible and put a stop to their ability to continue committing evil.
Especially if someone was outside god-belief systems, how would they ascertain that it is actually god believers speak of or just concepts they agree with and put in "god's mouth?"
Based on Romans 1-2 we can say that their spirit will have an inner witness to the truth that is spoken by God's representatives.
Whether or not they are people who seek after Truth will determine whether or not they respond to that Truth.
Using scripture is only a confirmation bias but doesn't address the question.
You are engaging in fallacious reasoning by trying to claim a particular book cannot have any value as a reference without giving any valid reason why your claim would be true.
Callling it "confirmation bias' doesn't constitute a valid logical reason because you've given no reasons why that automatically qualified as confirmation bias nor any reason why that would mean it's not true or useful to look at a particular book as a reference.
For example:
Using a math textbook to address someone's question of how an equation works is not "confirmation bias" just because they tell you it has the answers in it. Nor does that mean the answer in the book isn't true just because you think the are biased to look there.
So demanding someone answer your question without referencing a math textbook makes no logical sense because you've established no logical reason why they shouldn't.
In your case, you've established no logical reason why we shouldn't look to the Bible for an answer to your question.
For all you know it is the book that has the answers you are looking for, much like the mathbook you tried to refuse using in that example.
