Right, and there can be Christians who sacrifice to Ba'al and deny the resurrection of Jesus.
It's not a question of who's "call" it is. It's the objective answer to "Are they behaving in accordance to what the plain text of the Qur'an says?":
Indeed, Allah has purchased from the believers their lives and their properties [in exchange] for that they will have Paradise. They fight in the cause of Allah , so they kill and are killed. [It is] a true promise [binding] upon Him in the Torah and the Gospel and the Qur'an. And who is truer to his covenant than Allah ? So rejoice in your transaction which you have contracted. And it is that which is the great attainment.
Your examples is ridiculous, because people who would deny the resurrection would not qualify by definition as Christians.
What about Jews who aren’t Kosher? Or Catholics 60yrs ago who ate meat on Fridays & used birth control. Is it your place to say that they aren’t really Jewish or Catholic ? And what about Christians who pick and choose which parts of the Bible they believe ?
I worked with a woman whose husband is a missionary...he believes every word of the Bible is God’s word as written - Old Testament & New. In other words, he doesn’t believe that a story may be an allegory or parable. Are people who don’t, not Christians?
It’s not for me or you to say.
Your Quran quote doesn’t define islam or what is necessary to be muslim. There are millions of muslims who don’t even follow the basic Five Pillars of Islam - they may pray 3 times a day instead of 5 or not fast as they should during the entire month of Ramadan.
It’s not for anyone else to define them as ‘not really muslim’.