Christian Redditors: Is everyone a sinner?

My Christian friend tried to convince me yesterday that Christ is the only way for all humans to reach their salvation/heaven. Here is a piece of our conversation. Please answer my questions if you found them relevant.

• Me: What is the core principle of Christianity?

  Friend: Repenting. For all our sins. So that we may be forgiven.

• Me: So does this mean we have all sinned? Even a child?

  Friend: Yes, we all sin by not including God in our thoughts and actions. Only repentance will overcome this.

• Me: Is God all powerful? and if he is why does he need a mere human to behave in a certain way? After all he created people, so he can make them behave in whatever way he wants. Why give them a choice of action and then punish? Isn't this like putting mice in a cage and watching? What is God's benefit in doing this?

  Friend: I am not sure. But mostly it is because God knows that humans have another side which makes them work opposite to what he intended.

• Me: So is this what Satan is?

  Friend: This is the work of Satan.

• Me: What about non Christians (the billions of them). Did God create them? If he did not, then does this mean that God is also religious and only creates Christian human beings?

  Friend: I have no answer for that.