The problems of the Islamic world today are caused by Muslims who make wrong interpretations of the Quran, said Tun Dr Mahathir Mohamad.
Citing the Islamic State (IS) militant group as an example, the former prime minister said many Muslims tend to be selective in their understanding of the Quran for their own agendas.
"Today, we find groups like the IS killing fellow Muslims in the name of Islam. They shout 'Allahuakbar' or Allah is Great before killing other Muslims.
"Killing is forbidden in Islam and is certainly not in the Quran. These things happen today because we do not practise Islam the way it is taught in the Quran," said Dr Mahathir in a talk on "Islam and Islamisation" at the Perdana Leadership Foundation here yesterday.
He said the "glory days of Islam" when Muslim scholars thrived were long gone as Muslims had strayed from the true teachings of the Quran.
"The Quran teaches us all the good things and if you follow them, you will never go wrong.
"The early Muslims followed it and therefore they succeeded in life. That era produced those great Muslim scientists and mathematicians, but not any more.
"Now, we only have mathematicians who download and own 30,000 (images and videos) of child pornography," said Dr Mahathir in an apparent reference to Malaysian student Nur Fitri Azmeer Noordin who was jailed in Britain for possession of child pornography.
Dr Mahathir also said fatwa or religious edicts were merely "opinions" and not Islamic laws that every Muslim had to adhere to.
"But in Malaysia, you can be dragged to court if you go against it. For example, someone once issued a fatwa that Muslims cannot wish 'Merry Christmas' to Christians. I have been wishing 'Merry Christmas' for many years.
"But after the fatwa was issued, many people did not dare to wish it because they think it is haram."
On another matter, Dr Mahathir attributed the Goods and Services Tax (GST) and the rising cost of living as among the factors that accounted for lower Barisan Nasional votes in the Rompin by-election.
"I am not a factor. The people in Rompin are more concerned about issues like GST and cost of living. Issues like 1MDB don't mean a thing to village folk as they don't know what it is," said Dr Mahathir.
Barisan Nasional candidate Datuk Hasan Arifin won the Rompin seat by a 8,895-majority against PAS' Nazri Ahmad, a reduction from 2013's 15,114 majority.
Dr Mahathir also brushed aside allegations that he was responsible for the "missing billions" in the Bumiputra Malaysia Finance Limited (BMF) scandal during his tenure as prime minister.
He said the money that went missing "belonged to the bank" and not the country and he should not be blamed for it.
"I did not steal any money. No money went missing. What went missing was money belonging to the bank. I am not a bank manager nor the bank adviser," he added.