Epistle of James is pretty clear on this (and this will upset many modern Christians), but “even the demons believe”, faith without works is dead, and “a man is justified by works, and not by faith only.” — this is the line that Luther famously tried to remove from the German translation of the Bible. Man is saved by faith thru grace, but more specifically in Scripture man is saved by Christ’s perfect faith, not our own. Faith is best understood as “faithfulness” (as in marriage) and not mere intellectual agreement (which is what James is clearly dismissing). There’s a ton of verifiable historical evidence for the person of Christ as well as fulfillment of what He said would happen, as well as the miracles of his ministry and around the church. And plenty of saints such that we can have assurance are “in heaven”, but you’re right that the deeper mysteries of salvation as it effects you personally is not as clear, although in the negation it is — everyone knows what kind of life they could lead to not be saved. And if we’re very honest with ourselves we know the kind of life we should be living, what we’re called to, and that Christ is knocking.