Hospitality was an important concern for the ancient Greeks, and one of the main themes of The Odyssey. Not only was it customary for rich men, princes and kings to entertain each other and exchange gifts, it was considered the duty of every Greek to offer hospitality to a stranger in need. For travellers and sea-faring people, being able to rely on the kindness of strangers was particularly important. Hospitality was also considered vital on a religious level; not only did mortals have to worry that a stranger may be a god in disguise testing them, all suppliants also came under the protection of Zeus. Homer uses both guest-friendship and gods in disguise as recurring themes in his poem, a moral lesson for his audience about how Greeks ought to behave to each other. In The Odyssey, it is this hospitality, or lack of it, that marks the difference between civilisation and barbarism.