Edible Words

In his first preface to communion in the book of common prayer, Thomas Cranmer writes this about the communion meal: "...[it] being so divine and comfortable a thing to them who receive it worthily, and so dangerous to them that will presume to receive it unworthily"

It might seem strange to us today that eating bread and drinking wine could be divine and comfortable for some people and dangerous for others. How are we to be worthy to eat?