What is a "True Buddhist"?

I was reading a book titled "Buddha Dhamma for Inquiring Minds" by Ven. Buddhadasa. In this book he says ordinary people are Buddhists in name only and "**To be a ‘true Buddhist’ is to be ariya**, those of noble practice who understand everything around them correctly to a far higher degree than ordinary people." He also says "The reward you will reap with nobility is to rise from the level of ordinary commoner to become a **true Buddhist in the ariyan discipline**." Does anyone know what he means here? I thought a true Buddhist was someone who took refuge and sincerely follows the Buddhist path. I know the ariya sangha, including stream enterers, are at a higher level and are more knowledgeable, but isn't it wrong to say only they are true Buddhists? If it was just some random person saying this I would simply ignore it, but Ven. Buddhadasa is renowned and highly respected, so I take what he says seriously.