We know with certainty that the facts about God and Christ, as testified in Scripture, are true by revelation, and we accept (or embrace) that truth by faith. We can choose to accept by faith what God has revealed, or we can choose to deny by unbelief what God has revealed.
Believers ought not to buy into the world’s definition of faith. Faith is not choosing to believe what you cannot know with certainty; but rather, faith is knowing with certainty what you cannot prove to others. Millard Erickson, Christian Theology (Grand Rapids: Baker, 1990), p. 247, explains the need for the witness of the Holy Spirit:
There are a number of reasons why the illumination or witness of the Holy Spirit is needed if man is to understand the meaning of the Bible and be certain of its truth. (Neither the church nor human reason will do.)……The final reason the special working of the Holy Spirit is needed is that man requires certainty with respect to divine matters. Because we are concerned here with matters of (spiritual and eternal) life and death, it is necessary to have more than mere probability. Our need for certainty is in direct proportion to the importance of what is at stake; in matters of eternal consequence, we need a certainty that human reasoning cannot provide…
Those who do not have this faith cannot understand it. They see faith as some sort of “leap in the dark,” composed of a strange mix of insufficient evidence, doubt and optimistic hope. But true faith is not a leap in the dark. True faith is stepping into the light that cannot be seen by the blind unbelievers (well, that’s not exactly accurate… at their deepest level the unbelievers know the light is there and they hate it, so they keep their eyes closed so that they won’t have to deal with it or acknowledge its existence).
Certainty is materialistically understood in an empirical sense of that which we can validate with the senses—or scientifically. The materialists also define a fact as that which can be verified. But the Christian has access to incontrovertible evidence that the unbeliever denies himself access to. The spiritual is just as real as the physical—the existence of God is as much an objective fact as any other fact. The difference between spiritual facts of reality and physical facts is not that spiritual facts cannot be objectively proven, but only that such proof will not be given until Judgment Day (or, after one dies)—not a single skeptic will stand before God and deny His existence. Until then, what we have is not really an inability to determine with certainty that God exists; but rather, a denial of the spiritual as invalid, unreal and unacceptable, coupled with a demand that for materialistic proof.
Philosophy will gladly tell you that all sensory-based evidences are filtered through your own mind (and are therefore as unreliable) as much as any spiritual evidences, such that you cannot be certain even of your own existence… Except that you cannot validly deny your own existence. And those of us who have embraced God’s revelation of Himself to us cannot validly deny His existence any more than we can deny our own.
Because God does exist, and because He is both omnipotent and Spirit, God is able to reveal His truth to men in spiritual ways that are just as valid, just as real, and provide the same (or greater) certainty as any physical evidence of any other fact ever can. It is a mistake to think that because the minds of men are unreliable, then such communication of truth from God to men is also unreliable. Between the recipient and God, the spiritual facts are utterly certain—not of mere confidence (which may be justified or not) but of evidence—even though such facts cannot be proven with certainty by the recipient to other men. You see, other men may consider the recipient to be unreliable, but the recipient rightly knows that God is reliable, because God communicates the certainty of the truth with the truth.
However, God has not left us with only personal revelation. Along with personal revelation, God has left us His word, the Holy Bible, as the standard of His revealed truth. God spiritually reveals that Scripture is His inspired and preserved word, and He personally reveals only what is revealed in that Scripture, making the Bible—and not personal revelation—the standard of truth for the Church.
Ken Hamrick, 2013