Ok, let me put it in a way that you might understand:

• Person A: "You aren't an Atheist if you believe in God."
• Person B: "But I identify as an Atheist and I believe in God."
• Person A: "Then you aren't an Atheist."

You: "No true Scotsman! Anyone who calls themselves an Atheist is an Atheist, no matter if they believe in God."

Do you see how this makes no sense?

An Atheist is a person who doesn't believe in God, not a person who calls themselves an Atheist. And saying you aren't an Atheist if you believe in God isn't a fallacy but just purely the definition of the term.

Here's the Wikipedia definition of a Christian:

A Christian (/ˈkrɪstʃən, -tiən/ ⓘ) is a person who follows or adheres to Christianity, a monotheistic Abrahamic religion based on the life and teachings of Jesus Christ.

(Source: https://en.wikipedia.org/wiki/Christians)

So someone who does not follow or adhere a religion based on the life and teachings of Jesus Christ is not a Christian. Not by fallacy, but by definition. And it doesn't matter what they call themselves.