
"Talent" isn't some linear thing where if you're talented you make objectively good music and if you're not then you make objectively bad music, though. The only kind of talent you can quantify is technical ability, and, as anyone with half a brain and experience of Dream Theater knows, technical ability doesn't automatically mean good music. Therefore, the actual process of song writing (i.e. the melodies and rhythms used, and the manner in which they are arranged) must come from another source; how else do you explain, for example, that some musicians create simplistic, happy melodies, while others create forboding, complex ones?
The answer is that music (and in fact all art) is a reflection of the attitutes, beliefs and values of those responsible for its creation; in other words, their ideology. Before you try to dismiss this by saying "they just make music they like to hear, mang!" consider that it would be ridiculous to believe that different individuals' musical tastes just appear out of nowhere; obviously something has to influence what sounds they find pleasant, and which they find unpleasant. I believe that people seek out music which reflects their own ideology. This certainly explains why so many metal fans (especially in subgenres such as black metal) are socially frustrated, angry and alienated; metal is socially frustrated, angry and alienated music, so these sorts of people gravitate towards it.
With that established, it should become evident why Opeth becoming Christian would be a concern; their music, being a reflection of their own ideology, would be in for a drastic change if their ideology underwent a change so (presumably) radical as becoming Christian. And, given the ideology of many of Opeth's fans, this change may cause the music to become unpleasant to their ears.