Knowledge of God -- especially saving knowledge -- is not attainable directly by human reason. We are finite; God is infinite. We are fallen and sinful in our minds; God is perfect. There is a great gulf or barrier between us and God, if there is a God.
"Natural theology" does not take knowledge far enough. Science describes what is; it cannot prescribe what ought to be, i.e. ethics. Science gives wonderful descriptions of the history, composition and structure of the physical universe, but it doesn't answer the questions of meaning, purpose, values, and significance that we as humans really need. It may be valid, but it is incomplete. Unless God reveals himself from outside nature, we can have no authoritative knowledge of God beyond some vague speculations.
Christians claim that indeed God has revealed himself to mankind, in the Bible, and especially in the Gospels of Jesus Christ. But it is up to each one of us to decide to believe the testimony recorded in the Gospels. Such a choice is not irrational, because the Bible is rooted in history, but ultimately believing in Christ requires a personal act of faith that goes beyond what can be known by reason.