When I ask what it means to believe in God, I am really being superfluous, because it is impossible to say what it means to believe in God without first answering what it means to believe. Stating it means to believe something is notoriously difficult. One hypothesis is that beliefs are thoughts about facts that occur to us in the form of sentences. For Descartes, thoughts that weren’t expressed in language were not thoughts at all, though they may be passions or feelings.
The first objection, though, may be that not all thoughts or beliefs are actually expressed in sentences but that they could be. For example, most everyone believes that a regular-sized automobile is larger than a normal basketball, but few people ever express that belief in the form of a sentence. It is averred that someone holds the belief if they would answer “yes” when asked whether a car is larger than a basketball. We might complicate things by asking whether a dog would believe a car is larger than a basketball, and it seems many dogs act as if they believe cars are bigger than basketballs, but they can’t express it in a sentence, even when queried.
So, is it enough to “act as if” something is true to substantiate belief in that something? Back to the original question, can we say someone believes in God if that person acts as if God exists? So, we might say someone believes in God if we see them praying, avoiding sin, or something else. On the other hand, we might run into serious conflict. Most people claim to believe in God and that God will provide a blissful afterlife. In other words, they express these beliefs in sentences. Their behavior, on the other hand, tends to reflect a general dread or terror of death or the afterlife. The behavior of many but not all self-proclaimed believers would indicate that they think death is the finality of life or the beginning of an awful punishment rather than a reward for a life well led.
Or, perhaps, these same people sincerely proclaim their belief in God, but their actions reveal a hidden belief that their lives have not been properly spent.