Basically, we can either claim that God can be known through reason alone (Samuel Clarke, Anthony Collins, Voltaire, Kant, Nyāya, Śaivasiddhānta…) or that S/He can be known through personal insight and/or Sacred Texts (Śrī Vaiṣṇavas after Yāmuna, Gauḍīya Vaiṣṇavas…).
The first attitude could lead to a rational theology, or even to deism, as it happened during the Enlightenment, since it presupposes that all human beings could equally reach an adequate knowledge of God, if only they were to apply properly their instruments of knowledge. The second attitude suggests that there is no way to prove the existence of God apart from one’s own experience and the (one of others as recorded in) Sacred Texts. Historically, religions tend to explain that one should be suspicious of one’s insight alone, since it might be due to one’s projections of what one would like to see as the ultimate reality, so that the second way is usually at least a mixture of personal experience and its validation through the Sacred Texts. In other words, if you “saw” the Virgin Mary as she is described by other reliable sources, yours could have been a genuine religious experience, whereas if you “saw” a Flying spaghetti monster, you should reconsider your evening meals.
Now, the first attitude may seem appealing to our contemporary and secularised world (csw), since it does not recur to the external authority of Sacred Texts, which is usually hard to accept to members of csw. However, it presupposes a totalitaristic move, since if the existence of God can be rationally proved, then your failure to comply amounts to a cognitive fault. It is the same argument which enabled Soviet Union (etc.) leaders to claim that the ones who were not agreeing with the rationality of Communism needed to be reeducated.