I don't believe we should live a lie..If you have to make believe you are in harmony with their faith..that is wrong..If I am strongly opposed to a faith and are in a situation to converse, I would give my basis for believing or not...
Be up front, if you don't agree with something..state it, they may still want you to be a part of the service,and who knows, maybe you could persude them to what you believe..