I don't believe that idealism or naivety are essential traits of Christianity. It's just that from interacting with Christians in person and online, I get the general vibe that the culture encourages thoughtfulness and frowns upon people who refuse to justify their treatment of others.
I have also noticed a huge variation among Christians regarding how they approach conversations involving the supernatural. Some Christians are very very careful with what they will claim and the position they defend, avoiding making concrete statements about exactly what God or heaven is like. And some Christians are completely unabashed in saying things like "oh yeah, God is literally a thinking feeling person who watches over people with his two eyes. Something bad happened to you? God was probably pissed at you for some reason, or maybe it would have been much worse if god hadn't literally reached down from heaven to interfere for the sake of preventing a worse inconvenience for you personally."
That's why I say that some impressionable idealists are only Christian because they see the culture of morality and just assume there's probably something correct within the Christian religion, and not because they find the metaphysics of creation particularly compelling. That's the general phenomenon thing that I see as naive.