Christianity as a belief system is strongly focused on believing as a matter of... loyalty, "keeping the faith" so to speak.
This sets up a discordant situation between believers and disbelievers; it's probably a mistake to locate any resulting 'hostility' within one group or another.
They represent threats to one another's world-views, and serious threats at that. The disbeliever can muster the contemporary prestige of scientism, the history of centuries of established "Christianity" and its misdeeds, plus abundant evidence of what's called "The Problem of Evil" at work everywhere on Earth. The believer can't honestly deny these things, but can't let them overwhelm his trust in God's power, wisdom, and benevolence.
The fact that he continues to resist the disbeliever's "natural" resolution to the difficulty is bound to appear "irrational" to the disbeliever, who may not wish to be impatient and patronizing about it--but the alternative, to seriously consider that the believer might own some incredible truth, is quite impossible, and makes no sense to him, and yet he's being expected to take it seriously. The only way that this can look to him is that his perfectly reasonable view of things is being assaulted by victims of a contagious (and repellent) delusion.
Wow! A certain amount of resentment--for having to deal with all this nonsense yet again, seems perfectly natural! ("Why do they keep trying to push this stuff on me!") The unlikely truth--that it's the "reasonable" people who happen to be mistaken--is precisely what a "reasonable" person will naturally reject!
And then there's that "inside" factor. The disbeliever too is a spiritual being, and can not find any lasting happiness without giving his spiritual nature its due! A believer will of course want to help him with this difficulty, and the resulting conflict is not one to be easily resolved.