Well, let's try an exercise to evaluate this idea.
Let's pretend that God does exist. No fair arguing against his existence for the sake of this exercise, because it's just an exercise.
Let's say he is a powerful spiritual being and that he created everything (icluding setting up the laws of science, physics, etc) out of nothing. He exists in a spiritual realm that is unfamiliar to us as humans. And let's say he can affect reality by willing it. He can create, destroy, and alter physical, mental and spiritual reality, simply by willing it. Also, let's say he can influence people in their hearts and thoughts.
How do you suppose he can be tested and proved by his creation?