Arelith

So I was thinking about playing a character who believes in the Rising Sun Heresy, but one part of the belief system confuses me. The sourcebooks mention that the Brotherhood of the Glorious Sun has been tolerated for a long while in the church of Lathander, they even taught that Lathander was a reincarnated form of Amaunator. But the Risen Sun takes it a step further into heresy.

What does that mean? Is the core belief that Lathander was about to transform and Amaunatorcs return is near how they took it a step further into heresy?

It's still a non-standard form of worship. Heresy doesn't mean "wrong" or "widely condemned" by its own definition, that's just a very common reaction to many heresies.

In this instance, it's a representation of a non-standard belief that's tolerated within an existing Church. There's plenty of scholastic differences between many ecclesiastical traditions of the same denominations. This one's just not as extreme as, say, believing Shar and Selune are the same thing, or that Waukeen is actually a shell corporation for a demon lord.