MAGA MAN
Let's go Brandon!
For starters, many Christians adhere to the OT. But regardless, you're taking that phrase from the Koran out of context, just as someone who wanted to make a "case" against Christianity could do w/ the OT.
That line you reference was a specific situation in their history, and referenced a group that had declared a war against followers. I don't proclaim to be an expert on Islam, but neither are you - and context is everything w/ any text like that. It's unfortunate that it gets posted to some websites and repeated as though it's a tenet that all Muslims follow in all situations.
Again - clearly, they do not. Use some logic.
A "Christian" that adheres to the Old Testament is a Jew. And Jews, more or less as a rule, know the history of their text and are not antagonist against non-followers of their religion.
Yet a large faction of Muslims are, therefore, irrespective of what your 'Koran experts" tell us, believe as I do, that the Koran commands them to do violence until there are no non-believers of their religion.
Until you and other leftists recognize that, there isn't any way to come up with a solution.