Righto Alexa, it is the content of his songs and arguably the music itself. Cat Stevens, now known as Yousef Islam, sings Islamic Nashheds but without music (other than a drum, which is permissable in Islam).

I doubt you will get a lot of muslims on here bashing the guy for converting. You may get that later based on his actions after his conversion.

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"Diplomacy is the art of saying 'Nice doggie' until you can find a rock - Will Rogers"