This Lamp has a great discussion on how to translate perhaps the most important word in the New Testament, πιστεύω/pisteuo. It’s a lengthy read, but well worth your while.

THIS LAMP :: Has Faith: John 3:16 in the NEB/REB–Good Translation or Not?

Well, the problem is with our English word. It has two different meanings. Believe can mean to accept something as true. But believe can also mean to hold an opinion. When John speaks of those who believe in Jesus in John 3:16, is he speaking of the same kind of belief when we say, “I believe it will rain tomorrow”? Of course not. Such belief has to be more than opinion. It also has to be more than mental assent. James foresees this as a problem when he writes,
“You believe that there is one God. Good! Even the demons believe that—and shudder.” (James 2:19 TNIV)
Believe may simply not be an adequate word for πιστεύω in English. It’s awkward, but the Amplified Bible gets the meaning across fairly well with “..so that whoever believes in (trusts in, clings to, relies on) Him shall not perish…” The parenthetical definition for believes in–”trusts in, clings to, relies on”–gets it right. But the Amplified Bible is not really suitable for any kind of use in a group setting (I don’t really even recommend the Amplified Bible in general), so how can πιστεύω in John 3:16 best be rendered?