Why do Christians tell me I’m going to hell?

I’m not Christian, im an atheist and I’ll take this down if im not allowed to post here, but I do have a genuine question.

Whenever I tell someone im an atheist, they respond with some form of “you’re going to hell” and I wanna know why that is? They always preach about how their God is forgiving, so wouldn’t He be able to forgive atheists for not believing in him in a world full of different beliefs if he did exist? Why would good people be sent to hell just because they don’t believe in Christianity? The same thing happens sometimes when I tell someone im pansexual or transgender (ftm), and I thought the religion was about being loving and accepting. I think it’s amazing when people find what works for them in terms of beliefs or identity, why do they feel the need to tell us we’re going to hell?

Again, I’ll take this down if im asked to and I’m so super sorry if any of this is offensive, I just would like some answers :)