Going off the Freeport Trilogy description of the founding of the faith it seems a more hidden knowledge religion than one based on spreading all knowledge.
Remember that Vecna is a god of hidden knowledge too so some appropriate things could be taken from there and adapted.
From the Living Greyhawk deities document 2.0:
Vecna's worshipers always whisper their prayers. True rites are never performed in front of those of other faiths.
If the god knows all the true hidden knowledge about souls then he could be useful in evaluating the dead so they can go to the appropriate afterlife, similar to Maat in Egyptian mythology.
Lore of the Gods has some good stuff for this:
Maat’s rituals revolve on ceremonial purification. During
ceremonies petitioners are often bathed and perfumed as a sign
of cleansing. Incense and oils are also common elements in these
ceremonies. A common prayer to Maat is as follows: “Upon my
heart that I have had when on Earth, don’t stand against me as
witness, don’t make a case against me beside the great god”.
Thoth is the counterpart of Maat recording the lives of men for their eternal judgment. He is more directly linked to knowledge
Thoth is the giver of knowledge, and his
followers petition him for mental strength and clarity of vision,
especially in the face of difficult decisions. Scribes and other
writers pray to him for inspiration and better command of the
art. Others pray to him to settle disputes. Finally he is given
sacrifices to be kind when recording the events of one’s life in the
book of eternity and to petition on behalf of the faithful should
their final judgment be in question.
Thoth’s cult is one of profound and intense ritual
interjected into almost every aspect of their daily lives from
dressing, to eating, to entering certain rooms. The most elaborate
rituals involve major events such as birth, death, marriage and
indoctrinations. Their initiation ritual requires years of intensive
training to complete, following which the initiate is laid in a
sarcophagus and pierced through the head with a spiraling
beam of white-light that links his will to Thoth and projects it
into higher consciousness.