In a guest all Gods are united. This is what the Shastras declare.
48. (A guest) should not be asked his family or his clan ; [Lit., his gotra] he should not be questioned as to learning or knowledge. [But
the host] should take him to be a god himself; for, in him are united
all the gods.
(Parashara Smriti, Chapter 1)
Further verses depicting the importance of guests (from Parashara):
45. Where a guest goes back from a person's house, ' – his hope of a welcome unfulfilled, – the forefathers of the householder do not eat,
during the period of ten years and five.
46. Where a guest is disappointed in his hopes – useless are the burnt-offerings made by the host, although he may burn a thousand
loads of wood and a hundred jars of ghee.
58. Those who do not perform the rite for the Visvadevam, or the rite of hospitably receiving guests, – they all are destined to go to hell,
and in the next existence are born as crows.
62. Whether it be a robber, or a Chandala (the lowest of all the castes), or an enemy, or a parricide, – any one arrived at the time of
the rite for the Visvadevam, is to be welcomed as a guest, who is like
a bridge for crossing over to the seats of bliss.
Similarly, the Manu Smriti says:
Manu Smriti 3.72. But he who does not feed these five, the gods, his
guests, those whom he is bound to maintain, the manes, and himself,
lives not, though he breathes
An interesting story, told in the Linga Purana, which emphasizes the fact that guests are always to be treated as God, is as follows:
In the secluded and thick Forests of Daruvana, select Munis including
Sapta Rishis and Maha Yogis were engaged in deep Tapasya along with
their wives and children. As the Munis were busy with their meditation
and Yagna Karmas, the wives who were all Maha Pativratas were serving
them in their spiritual activities and family chores. Maha Deva
appeared in Daruvana to test the depth of the Rishis and their
Tapasya. He assumed the form of a semi-lunatic but of an astonishingly
handsome and shapely physique; as a ‘Digambara’ or naked, he was
laughing loudly some times and behaving erratically always. Soon he
became the talk of Daruvana especially the feminine gender. Even the
renowned Pativratas of leading Maharshis, let alone the unmarried
Kanyas and elderly women, spontaneously professed attraction and
infatuation for the Stranger. Parama Shiva in disguise who terminated
Kama Devata into ashes with his anger by his third eye had kindled
passion among the Rishi Patnis including of the Sapta Rishis! Some of
the enterprising women entered his ‘Parnashala’, declared love for him
and enquired of his antecedents. He smiled but would not reply.The
Maharshis reacted sharply against the Stranger as he was ruining their
family lives; they all confronted him one day and gave him a
‘Shaap’that since he provoked his wives with his nudity, his Linga
should be dropped down to the ground. Parma Shiva retaliated that
since no fault of his the Maharshis gave the ‘shaap’, he would not
give a return curse but his Linga would get fixed there as also
multiply into innumerable forms to burn off the areas of their
presence; further they (Sapta Rishis) would be hanging on the Sky
along with Nakshatras for ever! Having heard this the Sapta Rishis
were shaken up as the forest was gradually getting burnt off and the
fire was spreading far and wide. They approached Brahma at once and
the latter confirmed to the Rishis that the ‘Atithi’ or the Guest was
none other than Maha Deva Himself and that they were most unfortunate
in not realising the actuality. In fact Brahma reprimanded the Rishis
as they did not observe one of the fundamental Precepts of Dharma viz.
that a Grihasti or a Family Man ought to treat Guests as Devatas (Athithi Devo Bhava)!