According to Indian culture, Guests (Atithis) are treated as equal to the God.
Did this originate from Hinduism? If so in which text/scripts is this quote mentioned?
Athithi Devo Bhava, which as you said roughly means "treat the unexpected guest as a god", originates from the Taittirya Upanishad of the Yajur Veda. Here's the whole hymn that it's taken from:
After having taught the Veda, the teacher instructs the pupil: 'Say what is true! Do thy duty! Do not neglect the study of the Veda! After having brought to thy teacher his proper reward, do not cut off the line of children! Do not swerve from the truth! Do not swerve from duty! Do not neglect what is useful! Do not neglect greatness! Do not neglect the learning and teaching of the Veda!
'Do not neglect the (sacrificial) works due to the Gods and Fathers! Let thy mother be to thee like unto a god! Let thy father be to thee like unto a god! Let thy teacher be to thee like unto a god! Let thy guest be to thee like unto a god! Whatever actions are blameless, those should be regarded, not others. Whatever good works have been performed by us, those should be observed by thee,--
'Not others. And there are some Brâhmanas better than we. They should be comforted by thee by giving them a seat. Whatever is given should be given with faith, not without faith,--with joy, with modesty, with fear, with kindness. If there should be any doubt in thy mind with regard to any sacred act or with regard to conduct,--
'In that case conduct thyself as Brâhmanas who possess good judgment conduct themselves therein, whether they be appointed or not 1, as long as they are not too severe, but devoted to duty. And with regard to things that have been spoken against, as Brâhmanas who possess good judgment conduct themselves therein, whether they be appointed or not, as long as they are not too severe, but devoted to duty,
(1-7) Thus conduct thyself 'This is the rule. This is the teaching. This is the true purport (Upanishad) of the Veda. This is the command. Thus should you observe. Thus should this be observed.'
The quote is from Verse 2, where it's translated as "Let thy guest be to thee like unto a god!"
The mantras are from the Shikshavalli of Taittiriya Upanishad, Yujur Veda. It is part of the famous four phrase code of conduct.
मातृदेवो भव । पितृदेवो भव । आचार्यदेवो भव । अतिथिदेवो भव ।
matrudevo bhava | pitrudevo bhava | acharyadevo bhava | atithidevo bhava
Shikshavalli partly includes advice on ethics and morals given to graduating students from a Vedic gurukul (school) and their responsibilities after graduation apart from prayers and philosophical instructions.
The word Atithi means Na (No), tithi (Dates). So it means unannounced (unexpected) guest.