A God who knows how to create a universe will also know how to keep a proof of his existence in the created world. And he will also know how to make his presence known to human individuals.
Now let us suppose that this God has created the universe but that he has failed to keep any proof of his existence in the created world. Neither has he ever revealed his presence to anybody. In such a case no one will ever know that there is such a God and man will believe in as many false gods and goddesses as possible as they have done in the early ages of human history.
Now let us suppose that God has created the universe but that while creating it he has totally forgotten to keep any proof of his existence in the created world. However God has regularly revealed himself to many human beings in all the ages of human history. These human individuals through their personal experience will come to know that there is such a God with such and such attributes and they will also know that all the other gods and goddesses that man has imagined so far are all false gods and goddesses only. In this way human society will slowly move from polytheism to monotheism.
But this situation is not an ideal situation at all, because this will ultimately lead to authoritarianism and agnosticism/atheism/scepticism. The ideal situation is the one where there will be both; there will be the personal experience of human individuals and at the same time there will be one or more proof/s of God’s existence in the created world.
From above I hope it becomes clear that if there is a God at all, then why that God will have to reveal himself to human beings from time to time, as otherwise they will never come to know that there is such a God and as in such a situation they will believe in false gods and goddesses only. At the same time if God does not want to breed authoritarianism and agnosticism/atheism/scepticism, then he will also have to keep a proof of his existence in the created world.
I think I have been able to make my point clear that if there is a God at all, then why it is possible for human beings to personally know there is a God, because in his own interest God will have to reveal himself to man from time to time.
I personally know there is a God. That is why I also know that scientists will never be able to explain everything of nature by natural means.