Jms. 2:19 “You believe that there is one God. Good! Even the demons believe that – and shudder.”
To acknowledge the existence of God is to assent to a proposition even demons hold, yet no one would argue that acknowledgment saves demons from hell.
There are many who attend church occasionally or even religiously, never having been born into the family of God, with no concept of sinfulness within and no appreciation that their life is in desperate need of transformation. They believe in God. Big whup. The demons believe God exists; they at least have the good sense to tremble at him.
Believing in God is not the point. Believing God is the point. Believing him when he says I am a sinner in need of salvation and he has provided a Savior. Believing him when he tells me Christ’s death was payment for my sin. Believing him when he says the faith he gives is saving faith. Believing in God does not produce the changed life, the deeds James refers to in this passage. What does that is believing God and acting in that belief. It is an ongoing process when I believe him sufficiently to obey him and choose to align my behavior with his written Word. I am not always consistent in doing that, but it is the desire of my heart.