Religions have been appropriating each others symbols and celebrations since time and religions began.  As a Celt you could be equally offended that the Druids appropriated their symbols etc from those who preceded them.  Just because we don't know who these people were and what their beliefs and symbols were only makes it worse.  As Celts (I'm a Celt too, or at least I think I am) we don't really know who we are, a product of countless physical and cultural invasions and appropriations.