P. N. Oak published a variety of Hindu Revisionist books starting from 1989, declaring the Taj Mahal, The Kaaba, Westminister Abbey, and the Vatican, among other things, as Hindu monuments converted by others in the world. From my experience in India, a large number of believers of Indic religions believe in his theories, despite their contradiction by nearly every other historian. This is especially true with tour guides I’ve had.
How did P. N. Oak formulate his evidence, and why did that catch on so quickly? Is it being taught in Indian schools as history? Are there any examples of other historians publicizing similar revisionist history and catching on as belief within certain ethnic or religious groups?
Relatedly, was there truth to the claim leveled against Babri Masjid, that there was a Hindu temple also underneath that Mosque?
P.N. Oak's "theories" are, to put it mildly, not widely accepted by anyone outside of a small minority of Extremely Conservative Hindus. No qualified academic respects him or his work. He selectively uses and misrepresents sources to prove an extreme Hindu worldview incompatible with mainstream history.
Oak, however, is part of a broader movement called Hindu Nationalism or Sangh Parivar. Hindu Nationalism seeks to redefine India as a Hindu nation and assimilate or expell Muslims, Christians, and other people regarded as "foreign" as well as to present Hinduism as dominant above all other religions. In 1998, they took power through the BJP, who were formed as an economic and political moderate but still Hindu Nationalist party. They began to target Muslims in Gujarat and began a project of textbook reform.
Moving into post 20-year developments is against the rules, but I think I can talk about this since it falls under the banner of historiography, one of the consequences of the BJP's review was to implement Hindu Nationalist ideas and minimalize or demonize non-Hindu contributions to India in textbooks.
The Hindu Nationalist movement has its roots in Indian Nationalism and has been steadily growing since 1947, but boomed in the 1980's as a reactionary group. Faced with modern scholarship and increasing wealth disparity and rapid population growth, Indians became more conservative, especially in the more populous North. This movement defined itself in opposition to contemporary scholarship around the Aryan migration theory, promoting the Out-of-India theory that the Indo-Aryans were the original inhabitants of India as well as ethno-religious-nationalism and traditionalism. This appealed especially to poor but high caste induviduals and those in small villages but quickly spread.
Hardliners used flawed history to justify their positions. Oak was part of a wave of ethno-nationalist historians from the 80's and a part of this movement. His continued popularity can be traced back tho the 80's Hinduvata ideal and the rise of the Hindu Nationalists to political and educational power in 1998.
*Visweswaran, Kamala, Michael Witzel, Nandini Manjrenkar, Dipta Bhog, and Uma Chakravarti. 2009. The Hindutva view of history: rewriting textbooks in India and the United States. Georgetown Journal of International Affairs 10(1):101-112