Suppose a Lawful Evil Fighter worshiped a Lawful Good deity of war (for whatever reason... say, that the Fighter misunderstood the nature of the deity because he read garbled scriptures, for instance!) Now, the Fighter is utterly convinced in his heart that he is doing his deity's will, and he is completely devoted to his (mistaken) notion of the deity... so one day, the Fighter dies. Where will his spirit go? Remember, he's worshiped the Lawful Good deity all his life, so normally he would go to his deity's realm, but his alignment is Lawful Evil, not an alignment acceptable to said deity, so he would likely wind up in Acheron or another Lawful-Evil plane? If, for the sake of argument, the Lawful Good deity WANTED to accept the Lawful Evil Fighter into his realm, could he? Or would the petitioner wind up on the plane he belonged according to his alignment, whether the deity liked it or not? (that is, can a deity "save" a petitioner from his deserved fate, or not...?)
What happens to a character who worships a particular deity, but has an opposite alignment...?
I'd say that Deity trumps alignment in most cases but that doesn't mean the deceased is in for a nice time in Heaven. the nature of petitioners is to "become one with their plane" and this guy clearly has a long way to go! If he's a devout worshipper of the Power, then they might take him, but he soul is going to have to work off his spiritual debt.
The Power might let him do it in their realm (possibly in a place where devout but sinful worshppers go), or they might be sent to another plane for their crimes. To look at it in (very simplistic) Judeo-Christian terms, if you worship God but are evil, you go to hell not heaven; God says you knew the rules, now suffer the penalty for sinning. Depends how forgiving the Power in question is I suppose, and how its pantheon is set up: Many religions appoint a divine judge of the dead specificaly to resolve this kind of situation. As a side point, most Eastern religions use the idea of doing penance in hell, then moving on to heaven, rather than being banished their forever.
Oh, and the character probably knows he's been a bad boy anyway, becuase he's probably constantly been breaking commandments while alive. His holy book should say what happens to pople who sin against his power (worshippers of Heironeous might be condemned to his brother Hextor's realm, for example).
Well, of course if the character KNOWS he's doing things that offend his Power, then he expects to be punished! I'm talking about a situation where the would-be "follower" is genuinely unaware that his actions are NOT the will of his deity. Suppose the deity were Lawful Evil, and the character were Lawful Good (and believed the deity to be the same!) So the character is going around doing good deeds, etc. all the while oblivious to the fact that his deity is Lawful Evil and HATES good-deed-doing... when the character dies, what would happen THEN?
If you read the Forgotten Realms novel the Prince of Lies (not quite sure of the exact title in English) you can find this situation happening.
A warrior who used to be a follower of Torm (lawful good) and then has turned his back on him finally goes back to his deity's path and dies serving its will. Yet the problem is that it's not Torm's will he was following, but Cyric's (chaotic evil) who was impersonating Torm.
The guy then ends up in the Gray Waste. He asks for Torm's reprieve, but the paladin-god CANNOT allow him in his realm as long as the guy his not a hero in is heart, a real follower of Torm.
The story ends with our petitionner guy saving the day risking his life, and then he's accepted among Torm's faithfulls.
I guess that means a power really doesn't have the choice. Yet, maybe it depends on the pantheon. After all, evil Illithid fall into the lands at Ilsensine's, and nonevil berks end up in the Gray Waste at Hades' (who's evil himself)
I guess it depends on the way they work.
In the chines pantheon, no such mistake, since there is a judge of the deads dispatching them. In the greek pantheon, everybody has agreed to let Hades take care of the big most of the petitionners. In Faerun, since there is no central authority to speak of, I guess things are let in their natural state, so it could be that powers do not have a choice. In the other hand, cyric is the god of the dead, entitled to claim every soul that hasn't been faithful to any power. In the absence of such a god of the dead, I can't tell what would have been the fate of our petitionner-guy.
I read in first edition books that when a good god is worshiped by an evil person, a dark deity or powerful fiend responds instead.
Even, if an evil god or powerful fiend is no properly worshiped another evil god or powerful fiend can respond instead.
Demon Lords worship is based especialy on that idea: that non evil worsipers worsip demon lord. Cult of demon lord is presented as benign church of some nature deiety (in case of Baphomet) who come in time of dire need.
Anyways, goal is to atract the cadre of worshipers and slowly coruppt them into demon service.
Meanwhile worshipers provide demon with divine energy of their faith. It is good way to increase your power while you weaken your upper plane rivals.
EDIT: So I think it does not matter who you worship becaouse god you worship will allways get that portion of your faith no matter what is your aligment, after all folks all over the planes pray to good gods for blessings and to evil gods to pacify them.
As of what happens to individual worsiper of opposite aligment, I guess there is no easy anwser: after all it depends from god to god and from cosmology to cosmology.
But I guess that worsipers aligment would eventualy shift to match to worshipers aligments (In 2e, under clerical aligment listing there were alse worshiper aligment listing to illustrate what kind of peaople and personalyties usualy worship that god.)
Perhaps this is how the fallen deva and repentant fiends come about.
The petitioner was of a different alignment to his god, died, then found that they disliked their new "Paradise", and found a more fitting environment. Maybe.
Unless I'm mistaken, it is always a given deity's choice as to what happens to his/her followers, independent of said followers' alignment. This doesn't mean that the deity is forced to keep all of his followers in his own realm - he could have his own special hell for believers who fail to follow him "correctly," for example, or he could choose to reject the soul completely... it's just that it's always his choice as to what to do.