In the world of A Song of Ice and Fire trial by combat is a very common way of trial. But in the events of A Song of Ice and Fire this way of trial has been found wrong many times. One example is Tyrion found guilty even when he didn't murder Joffrey. So does everyone believe in trial by combat (that it is blessed by god so that the right person emerges victorious)? Or is it just a way for the stronger persons to escape their penalty?
There's not enough evidence in the books to say whether everyone believes in trial by combat (or at least, everyone who believes in the Seven; I think it's an Andal thing, at least in this form, perhaps people who follow the Old Gods have something else). What we do know is:
- It's an old tradition. Most people aren't going to speak out against an old tradition.
- Any noble who thinks they're likely to lose in a regular trial has the right to demand trial by combat instead. Therefore it's in the interest of the ruling class to preserve this option.
- It's meant for cases where the facts aren't immediately clear. Therefore it's going to be hard to say "Look, the trial produced the wrong result!" We know that Tyrion didn't kill Joffrey, but it's not that clear-cut to the characters.
- It's not really about the facts anyway, it's about the will of the gods, which is mysterious. That is, if I'm accused of stealing a bagel, and then I win my trial by combat, it doesn't mean I didn't steal the bagel; it means that either the gods approve of me stealing the bagel, or else they have some other punishment in mind for me, that they will take care of. So for someone who's a believer in the Seven, the trial literally can't produce the wrong result. If it seems wrong, well, so do a lot of things that happen in life and the gods are responsible for those too.