While there is only one true system provided by God for us to have fellowship with Him and salvation from eternal damnation, there are many other false belief systems which are not provided by God. Islam is just one of many false religions, which for those who seek to grief and quench the Holy Spirit might follow.
If somebody is antiChristian, they aren’t necessarily Muslim, although Islam is also a false religion worshipping a god other than the true God made available to us through faith in Christ.
I am so happy that you have the courage to express what you believe. I was raised at a time when your opinion was pretty much accepted as fact in our family.