In Hebrew (and likely Aramaic), the meaning of the phrase "I believe ..." meant that the speaker responds or is responding to that belief with some sort of action or change in their life.
In the OT writings, this seems a truism. But notice how that foundation is cracking with the Hellenization of Judea, and then Roman occupation. It has to be addressed at all in James, et al because it became a new problem for that group of people--particularly notice it in Paul's writings to communities separated from Jerusalem (the center of Jewish orthodoxy).
I believe this had to do with the shift from "belief" meaning essentially "belief in a person" (i.e. loyalty to a patron) to the more Greek concept of belief as ideological. OT Jewish thought didn't seem to have any concept of a Truth behind reality; or, if it did, it was a personal Truth inseparable from a personal Being.