Honesty....... and if what they are doing is not something you believe and it is a conscious issue you could always go, if you want, but tell him you won't be participating anymore then to listen, if you even want to do that.
So then the question is does he still want you to go?
Would he want you to practice something you don't believe at the present?
As a christian there are certain religious Catholic activities i would refuse to participate in even though it is a "christian religion" as my conscience says the bible really teaches something different in regards to certain things that are "Catholic".