The atheists seem to feel that my keeping only a subset of Christian beliefs is intellectually dishonest, and that Christianity is a monolithic, indivisible unit which can only be accepted or rejected as a whole. In dissecting it and choosing which parts I like I am not a "real Christian."
Interestingly, conservative Christians make the same sort of accusation. If I don't believe and profess every aspect (of their particular version) of Christianity, I am the worst sort of blasphemer.
Both of these extremes see religion as all-or-nothing, and those who decide for themselves exactly what they believe are breaking the rules. You can't just make up your own faith! It's not like some kind of metaphysical cafeteria where you can choose an omniscient god with a side of benevolence, you have to take the prix fixe meal.
What both of these sides don't realize is that everyone chooses what they believe.
Sure, they might have been born into a family that practices a particular faith. As a child they may have been flooded with a particular viewpoint, and even threatened if they say or do anything that contradicts it. Still, at some point they have to decide.
It could be caused by doubt, and they start to question the beliefs they've been taught. It also could be caused by something completely secular, like moving and needing to adjust to a new environment. There are thousands of possible decision points - marriage, graduation, personal tragedy, arguments, experiences - and all humans encounter many of them throughout their adult lives.
That's when, on some level, they evaluate what they see and think ... and make a choice.
If they're the introspective sort they might examine every little aspect of their belief system. They might discard some parts and keep others. If they're not so inclined to minutiae they might throw out the entire system in favor of a different one that better meets their needs. Both of these are perfectly reasonable behaviors. Some people are loners and others need a degree of external validation. There are also some who really don't care and want to focus on whatever is in front of them, and philosophically speaking that's ok too.
The thing is that none of those options - or the beliefs they lead to - excuse behavior that is harmful to others.
An individual who cherry-picks from one or more religions has no one to deflect blame onto. If they say, "This seems mean, but my belief says that it must be so," they know full well that they chose that belief. They're also likely to go on and question that aspect of their belief system.
However when someone adheres to a belief system "whole cloth", this situation does not change. They can say, "This seems mean, but this doctrine or that person says this must be so," but at some point they still chose the beliefs, the doctrine, the person. Therefore they must find it to be morally acceptable.
"This seems mean, but this doctrine or that person says this must be so, and I agree."
If they didn't agree then they'd choose a new belief system ... or cherry-pick the parts of their current system that they can accept.