I know how people who don't hold a belief about a God think. But I want to know how a Christian, who is currently believing thinks about this and what is their justification to think so.
Well...pretend it's 5 years ago?

That's the same answer I would have given when I was Christian--it's how I thought then, even though I then took an extra step to try to make it fit a Christian mold.
I see. I myself have never believed in anything that can be described as religious belief. The questions I was asking myself were. If one does not have morality within themselves, how they will understand the moral lessons from the book they chose to read? I imagined myself laughing madly at or experience joy about any mass-murder story in the Bible. At the end I decided that I must have morality, because I don't find any of it funny or pleasing.
The same could be said about logic. One can find logic inside the Bible and never exit that loop. However to understand that logic, one should already have the logic within. And logic within me said that what I read is illogical.
So, for me, to become a Christian it means I have to destroy my own morality and logic and substitute them for those from the book. I have not found any other explanation for doing so, other than having for myself eternal life (dubiously good thing). So, I had to destroy my relationship with the world around, get some questionable morality rules, only for my benefit. The next question was "is that moral"? You may guess what was the answer.
Having been inside the mindset, and now being out of it, and having spoken to a lot of Christians both from the inside and outside, I'd say that a lot of Christians
do use their own intrinsic sense of morality, based on empathy, but they often deny that they are doing it--even denying it to themselves. I say I tried to take an extra step, once I'd decided what was right, to justify that decision in light of a Christian mindset. But I was especially thoughtful, self aware, and philosophical. A lot of people do it in a less deliberate way, by instinctively getting their own lens of morality in between the Bible and their brain. That is, say they read a story where God commands genocide, they don't then walk away thinking that genocide is ok. They either completely distance themself from the story, to such a degree that they don't notice that God commanded genocide, or it doesn't occur to them that those people were real people and not just characters in a book, or they read the story as being so representative of a different time and culture that it has no parallel at all today.
I remember, shortly after 9/11, my church, rather innocently I think (my church was definitely on the left-leaning, peace and love end of the spectrum) had a reading from one of the several places in the OT where the Israelites rely on their faith in God and, thus, are able to massacre a city. My mom whispered to me that she really didn't like stories being taught where people are able to slaughter their enemy because of their faith in God--it's exactly what terrorists think. It had never occurred to me to consider the flip side like that. You know, so many of the stories we have about the ancient world involve wars and killing...if you want to, you can consider that part of the Bible just a backdrop to messages about how relying in God makes you strong, and how God is a faithful protector and whatnot, without ever considering the fact that you're talking about genocide or human sacrifice.
In short--people
do approach the Bible with pre-conceived ideas that, say, fidelity is good and love is good, and mass slaughter is usually bad...but they often don't realize they are doing that. They stay blind to the parts of the Bible that say God helps you slaughter babies, only notice the parts where God fits their preconceived ideas about what is good, and then turn it around and think that they are forming those ideas about what is good by extrapolating them from what they see in the Bible.