For nearly three centuries, Belintar was the God-King of the Holy Country. Belintar defeated and slew the Only Old One, cast him down, and received acknowledgement from all the major cults of the Kethala. And although he took many incarnations over the centuries, Belintar was the continuous ruler (more so than even the Red Emperor).
Belintar never created a “state” to govern the Holy Country. He didn’t need to. It was held together by him and his helpers derived their authority from him. Belintar protected the Holy Country (i.e. led the military), interceded with the gods (ie served as a supra-cult high priest), and adjudicated between groups (but rarely individuals).
As a god incarnate in the world associated with all the major cults of the Holy Country, Belintar was his own power base. If the Holy Country was threatened, he’d call up the militias and temples, appoint helpers to recruit mercenaries, and lead the army into war. This didn’t happen very often – Belintar was kind of like Dr. Manhattan and I think between 1320 and 1616, he faced only one serious military crisis. That was in 1605 when the Red Emperor led the Lunar Army in a multipronged invasion of the Holy Country. Although the Tarshite feint into undefended Heortland was a near complete Lunar victory, Belintar and his army routed the Lunar Army at the Building Wall Battle.
Remember that was against a full fledged invasion of the Lunar Army led personally by the Red Emperor. Praxians, Manirian tribes, the occasional sea troll gang – these posed little threat to Belintar and can be dealt with locally. Belintar built some fortifications to help deal with the Praxians, now home to the Marcher lords. And so on.
But that system all fell apart in 1616. When Belintar died and did not return, every arrangement that depended on Belintar’s power quickly collapsed. Inertia and duct tape held some things together for a few years, but when the Lunar Empire invaded again in 1619, these fell apart the moment they encountered resistance. The Queen of Esrolia made an alliance with the invaders, new political arrangements were made in Heortland, and Caladraland united itself under a new king. From 1622-1625, the Hendriki king served as a warlord for Esrolia and Heortland, but he was slain by Lunar magic.
Can the Only Old One be worshipped and does he grant spells? He can be worshiped and he does provide a spell to the Argan Argar cult. But he is now only in the Gods Realm.
As an aside, Belintar is not a superhero. He was a god incarnate in the mundane world. The things he did make even Harrek and Jar-eel’s deeds look mundane. The destruction of the Castle of Black Glass, the Lead Hills, rerouting the Creek-Stream River, raising the City of Wonders, and the Building Wall Battle are all truly godly deeds.
The best and most authoritative background for Belintar is page 66-69 of the Mythology Book.