esp if you’re one of the devout ones who think they’ve been really good
The bible considers it a sin, but sin doesn’t keep you out of heaven if you’re Christian, you basically just have to try and do better.
The Catholic church decided it was a mortal sin, and because you didn’t have time to go to confession afterwards, you would go to hell.
That’s a drastic oversimplification, but it is kind of the root of it.