Question
Do all Non-Hindus remain as Pretas after death?
Answer
Usana Smriti's Chapter 7 has the following verse:
It is laid down that Sapindikaran, S'raddha should be preceeded by the Daiva, (i.e., offerings made to the Deities). There one should invite the departed Manes and point out again (i.e., invoke) the deceased.* (17)
On this verse the translator/commentator (Manmatha Nath Dutta) says:
A deceased person is called Preta till the celebration of the Sapindikaran Shraddha , after which he is designated Pitri,
So, the answer to your question is a Yes, as per Hinduism (Hindu Scriptures). After death, the Jiva remains as a Preta till all the necessary rites are performed duly.
Sometimes, if persons had unnatural deaths, additional remedies are required to be performed for these souls to be relieved from the Pretahood and attain the Pitrihood.
Many of these remedies are mentioned in Satatapa Smriti's last Chapter:
For one dying of cholera one should treat a century of Brahmanas with sweet edibles. For one killed by fire sticking to the throat one should give away a dhenu of sesame. (48)
For one dying of a disease of the hair one should perform eight Krichchhas. According to this regulation one should perform the funeral rites for them. (49)
Thereupon being freed from the condition of a preta (dead) the gratified Pitris (departed manes) grant sons, grand-sons, longevity, health and wealth. (50) Here ends the [account of the] : fruits [of various] acts given by S'atatapa to his disciple S'arabhanga accosting him with humility, (51)
Therefore, according to Hinduism, anyone dead attains the Preta state. And, to relieve them from this Preta state, some rites are required to be performed, after which the Jivas are elevated to the Pitri state.
(NOTE:- This answer is from the perspective of Hindu scriptures. What happens to the non-followers of Hindu scriptures after their death is something that we shouldn't be concerned about on this site. We can't know that either unless we have knowledge about their rites and scriptures)
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