Mystara does not have god(s). But Mystara does have Immortals. Great mortals of the past (hero's & villans) who have obtained a higher state of being that has some similarity to Godhood, they are not all powerful, but are very close to it. They are not all knowing although they monitor the present very closely, and they only someitmes demand worship. The Immortals are what grants priests their spells. There are an unknown number of immortals, of various strengths.
Let me mention that a few cultures (like some of the Heldann and some of the Atrughi) call their immortals gods, but they are in fact immortals.
Some cultures honor only one immortal, others have a pantheon, some heirearchical others just a grouping of certain immortals jumbled together, some organized into formal churches or Orders (like the church of Karameikos, or the Heldannic Knights*).
*= the Heldannic Knights templar is probaly a bad example since they only venerate one immortal, Vanya.
A Note about Glantri:
In Glantri, Preaching of the Immortals is illegal. Until the plague struck, Priests (often called Clerics) were banned from Glantri and they are still closely scrutenized and discrimanted and unwanted (unless someone is sick or needs food or water). Glantrians believe that Priestly magic is barbaric and they focus solely on Wizardly magic. Asides from small secret illegal groups, the Immortals are not followed in Glantri. Oddley enough though, Glantrians often refer to Rad but they themselves seem unsure what it is but normally view it as a name someone gave to pure magic. Temples of Rad are tended to by Sheperds of Rad who are wizards and they are mostly places for quiet meditation rather than a spiritual experience. Most outsiders believe Rad to be one of the immortals and find this a very confusing contradiction. A common symbol for Rad: 3 hemispheres which connect to form a triangle inside.