We will always have to wonder if maybe the whole world is an illusion, maybe what seems good to us is actually bad and vice versa, maybe GOD wants us to do the opposite of what he says, and so on. This approach is unhealthy, unproductive, and leads nowhere. On the other hand, if we assume that GOD is good and trustworthy, we can on this basis move forward. We can assume that our senses, logic, and memory can be trusted to teach us the truth about reality (provided of course that we use them properly and are careful not to deceive ourselves).
We can construct philosophical arguments and formulate scientific theories based on observations and experiments, believing that these do indeed teach us truths about reality. We can trust our moral intuitions to guide us on the straight and good path. And we can trust the Bible to tell us how to worship GOD, draw closer to him/her, and maybe gain the life of the next world. There is no doubt that this second way is much more positive, fruitful, and beneficial; this is the working theory and way of life for humans since the beginning of history.
We all trust our basic cognitive tools, and although we are aware of their limitations and the deceptions they may fall into, we believe they can be overcome and come very close to the truth. The fruits of this belief are all works of human culture.
In other words, it is impossible to prove whether GOD is good or bad, trustworthy or a liar, but the belief that he is good and honest allows us to move forward and flourish, while the belief that he is bad and deceptive eliminates any possibility of knowing anything. Even those people who enjoy wallowing in philosophical skepticism and a sense of existential meaninglessness end up pragmatically relying on their senses, logic, and intuitions, and actually believe in some truth and values (even if they deny it); this is part of their tormented hero image. If one takes the consequences of an evil GOD to the end, then even existential philosophy itself becomes a ridiculous joke, another delusion.