In the third year of his reign, King Fahd is taking great pains to reaffirm his religious credentials as the worthy keeper of Islam's holiest sites here and in Mecca as well as an active propagator of the faith throughout the world.

His concern for Islamic issues appears to be very much part of the royal family's noticeable turn inward to deal with the kingdom's internal problems. One of these is containing the pressures on the House of Saud from the powerful and increasingly assertive Saudi religious establishment and from Iran's Islamic revolution across the Persian Gulf.

Fahd is constantly praising Islam and crediting the kingdom's strict adherence to religion for its economic achievements of the past decade. Longtime residents say he does this much more than his predecessor, Khalid, who died in 1982.

Another explanation offered by western analysts for his special attention to religious matters is a possible desire to overcome a less-than-pious reputation in his youth with the kingdom's ulema, or religious leaders, whose support is essential to his effectiveness.

In late October, Fahd came here to inaugurate one of the largest printing plants in the world, capable of producing 8 million copies of the Koran and 30,000 radio and video cassettes of it each year. It is officially known as the King Fahd Complex for Holy Koran Printing.

The Saudi firm Saudi-Oger, owned by Lebanese-born Saudi billionaire Rafiq Hariri, built the plant and facilities for its 500 employes for $285 million and will be paid $140 million a year to operate it. Eventually it will include a research center for the study of the Koran and the hadith, Mohammed's sayings and deeds.

The most distinctive feature is a gigantic stone Koran resting open-faced on a book stand high off the ground in a marble courtyard.

Fahd also laid the cornerstone for an expansion of the Prophet's Mosque, one of Islam's holiest sites, where Mohammed's body lies. It will accommodate 170,000 people. He also started expansion of the ancient Qubaa Mosque here..

A committee of 14 Moslem ulema and scholars has worked for 10 months to produce a "perfect," meaning error-free, version of the Koran, which Moslems regard as the word of God as revealed to Mohammed.

So far they have found 3,000 errors in the prepared text, according to Adnan Ansari, the plant's production quality control manager. Every dot, accent and character of the Arabic language has to be exactly in the right place before the new Koran is approved for publication.

When it is, other Korans in the kingdom are to be confiscated and replaced with the new one. Millions of copies will be sent abroad and given away or sold at such a low price that the Saudi Koran is expected to become the standard modern version for all of Islam.

The Saudi Koran also will be published in a dozen foreign languages, although Moslems consider only the Arabic-language version the authentic word of God. Another committee of scholars is working on an authorized English translation, the first of the foreign-language versions.