I grew up in the church and still attend, so I understand the desire to say this. I can’t count the number of times I’ve heard things like this from a pulpit. But you also have to remember that no one knows the heart but God. You don’t get to decide if someone is a Christian or not.
And, even more important in this context, whether a hateful Christian is a “real” Christian or not doesn’t matter. Even if you could magically tell who is a “real” Christian or not because you’re on the inside, people on the outside sure can’t. And, spoiler alert! you can’t tell either.