Before oil, the Persian Gulf was one of the poorest and most isolated regions on earth. The Gulf states had no paved roads, no electricity, no hospitals, and no schools beyond informal mosque instruction. The interior was nomadic herding and subsistence date farming. The coastal towns had fishing and small trade.
Mecca in 1930
Same spot today
Within a generation of oil being found, nomadic Bedouins who had never seen a paved road were living in air-conditioned apartments in cities that hadn't existed twenty years earlier. Because the oil paid for everything, governments didn't need to tax their citizens to do it. In most countries governments extract resources from their population and the population demands representation in return. In the Gulf, governments distributed resources downward and the population accepted authoritarian rule in return. The ruling families bought political loyalty and got it.
Saudi Arabia had always been conservative, but its brand of strict literalist Islam had spent most of its existence as a localized desert movement with no money and no reach. In 1973, when Arab oil states cut off exports to the West and prices quadrupled, the Saudi treasury filled with wealth it had no precedent for. The government built a massive clerical bureaucracy. It funded state-employed religious enforcers who patrolled public spaces controlling what people wore and how they behaved. It rewrote the national school curriculum. Then it spent billions exporting that same model across the Muslim world, building mosques, funding religious schools, and buying influence over Islamic institutions. Cairo is where you can see most clearly what that money was working against.
In the 1950s and 1960s, Cairo was the cultural capital of the Arab world. Women swam in public pools in bikinis. They wore miniskirts in the street. Egypt under Nasser, its president from 1956 to 1970, was explicitly secular and socialist. In one famous speech Nasser described a Muslim Brotherhood leader demanding all Egyptian women be forced to wear hijab, and Nasser's response, which got the crowd laughing, was that he couldn't even get his own daughter to wear one. Religious conservatism was the opposition, not the mainstream.
The 1967 Arab-Israeli war destroyed Arab nationalism as a credible ideology and left a vacuum Gulf money filled. Millions of Egyptians went to work in Saudi Arabia and Kuwait because the wages were many times what Egypt could offer. They came back years later more conservative, practicing a stricter version of Islam than they had left with. Saudi Arabia bought into Egyptian media and television networks, funding the conservative and defunding the secular. Gulf-funded charities provided the schools and clinics the broke Egyptian state could not, and the charity came with culture attached. Nobody passed a law. The social environment changed until the bikini became unimaginable. Women who had worn miniskirts in the 1960s were covering their hair in the 1980s and watching their daughters cover their faces in the 1990s.
The Persian Gulf went from subsistence poverty to global wealth in a generation. That wealth built cities, bought political loyalty, and funded the export of a conservative religious ideology that reshaped the culture of the Arab world. Cairo in the 1990s looked nothing like Cairo in the 1960s, and Saudi money is the reason why.