No, it's perfectly fine to have an opinion about an ideology, like: "I don't believe in the Bible/Christianity, etc.". A statement like that only makes a judgement about yourself which is of course your right.

If you dislike something, there's usually a fear underneath that. People who dislike Christianity on a deeper level are probably afraid that if everyone was Christian X would happen. Sometimes that's realistic, sometimes it's not, but expressing your fear is OK and usually helps people understand you better than if you simply say "I don't like X type of people".

When you switch your focus to a group of human beings and say "I don't believe Christians are...", you're judging others. When we judge others we're probably bias in some way because none of us have enough accurate information to uniformly judge the millions of unique people that identify with a race or religion.

Just saying "I" first doesn't mean you can then say whatever you want. If the target of your judgements is a person or group of people, you're probably not being honest with yourself about what you're uncomfortable with and projecting your fears onto a group of people. That's typically what racism is about.

Instead, it's usually more accurate to express your fears about a particular non-human target (like a law, behavior, etc.) and leave people out of it.