There are four primary religions in Geora. The first is a dualistic religion, which is the most prevalent. The followers of good revere Radiar (Pelor) as the god of light. They believe that the world is spinning towards a final battle with the followers of evil who are led by the god of darkness (Bane) who goes by many names. To evil wizards he is known as Yevlyn the Blood Mage. To orcs and other warlike races he is known as Dimatyr. But all know him as the champion of darkness. Adherents of this religion, known as Dynamism, acknowledge the existence of both the god of light and the god of darkness, believing there to be no such thing as neutrality. Everyone will be forced to pick a side in the final battle and while both sides predict victory, no one can truly say for sure. The Republic of Berandal tends to favor the church of Radiar, while the followers of light are outright banned in Johak.
The second religion is a relatively new religion of monotheists who worship a god named Olarius (Bahamut). Followers are known as Olarians (Draconians), and they preach vehemently against the existence of any other gods. For their part, Dynamists acknowledge the existence of Olarius but believe that he, too, will have to pick a side in the battle for good and evil in the end. Olarianism has grown quickly over recent years centered in the great and ancient city of Kale-Jerome where it is said the god Olarius once walked as a mortal. Olarianism is technically a lawful good religion, but the zealous followers of Olarianism have, on occasion, carried out terrible acts of wickedness in the name of their god with various inquisitions and purges. Worship of Olarius is allowed in Berandal but suppressed in Johak. Some Olarians speak of outright secession from Berandal to form their own pious nation in the western region of Tandelle.
The third religion is a nature cult devoted to the goddess Volnar (Melora). Volnar speaks to those who spend a lot of time in the natural world and eschews civilization. Her followers don't preach against civilization, they just prefer to spend their time away from it. Hence her faithful are largely in parochial communities and she has a particularly strong following amongst the poor and downtrodden. Many of her priests are druids and non-Volnari druids get along well with the Volnari druids.
The final religion is the vile and subversive cult of Dezmaeron, the Demon King (suspected to be many different evil gods or an avatar). The Demon King was banished from Geora eons ago after ruling the world under a particularly cruel and iron fist. A very small number of folk actually want to see Dezmaeron returned to Geora. Perhaps they think it is inevitable and want to be on his good side. Perhaps they are just insane. Whatever the reason, this cult works behind the shadows to try and return their dread lord to power.
As far as conversion goes Pelor will represent Radiar and Bane will represent Dimatyr/Yevlyn. The other gods will have minor temples dedicated to them but generally their worshippers will align themselves with either Pelor or Bane while some of the unaligned churches are considered not to have picked a side. All good and unaligned gods in the PH are considered allies of the church of Pelor except for Bahamut, Melora, and the Raven Queen (the last of which is considered to be a dead religion). The evil deities are generally considered to have aligned themselves with Bane, but many of them simply choose to go their own ways. Bahamut will represent Olarius, worship of a dragon god being a relatively new phenomenon. Worshipers of Bahamut won't necessarily be monotheists, but they will strongly support the worship of Bahamut to the exclusion of other gods, seeing even Pelor as not worthy. Avandra or Melora will represent Volnar depending on the situation, but usually Melora. Dezmaeron is not properly considered a god, but he might actually be Asmodeus or an incarnation of Bane. No one really knows.