Various methods and arguments have been used to help prove the truthfulness of the Christian faith. For example, there are the philosophical proofs of God's existence. These, while not proving Christianity particularly, provide a foundation upon which other proofs may then be added such as the reliability of Scripture and the reality of the life and accomplishments of Jesus Christ.
Sound arguments are not enough to save a soul. Salvation, according to Scripture, requires a new heart. Mankind's resistance to Christ is not ultimately intellectual, but moral. Nevertheless, good sound reasoning is valuable to refute the foolish arguments against Christianity that are often raised.
The fact that man is actually spiritually blind offers another opportunity to prove that what the Bible teaches about God, Christ and Salvation are actually true. Personally I find it one of the strongest confirmations of the inspiration of Scripture and reliability of God's Word. It is also a heart-breaking proof. Allow me to explain.
The Bible clearly teaches that unregenerate people are spiritually blind. That is, they cannot comprehend, explain or figure out even the most fundamental spiritual truths. Remember Jesus' words to Nicodemus "Are you a teacher in Israel and you don't understand these things?" Nicodemus was very smart. But he couldn't see the plainest truth of Scripture.
The Bible describes the natural man as "blinded by the god of this age (2 Cor. 4:4)." They cannot see the gospel even if it was placed right before their eyes. This truth is presented frequently in Scripture in different ways using terms like "blind" and a "darkened" mind and even "dead."
And it is this very fact that I have often, with admitted sadness, found to be true among unconverted church-goers and religious people. Have you ever talked to an unconverted religious person about the gospel? I'm talking about a regular church-goer, sermon-hearer, Bible reader. They have sat with their Bible on their lap hundreds of times and read the Word. They have listened to hundreds if not thousands of sermons. And yet - when such people are simply asked to explain the gospel they are at an utter loss for words. Ask them why they think they should go to heaven - they are perplexed and confused and uncertain.
The conversation usually heads toward the good things they try to do and the bad things they try to avoid. Ultimately, if asked if they are sure, the usual response is "no." They hope so. But it is hard to be sure of salvation when their understanding of it is based upon their works and their goodness.
I expect the irreligious person to be ignorant of the gospel. But when I ask a regular Bible-reader and church-goer to explain "what must I do to be saved?" and they have no answer - it convinces me beyond a shadow of a doubt that what the Bible says is true.
You see, the unconverted man can sit through the most moving and clear presentation of the gospel. He can hear the invitation to repent of his sin and trust only in Christ for salvation. He can be told about the cross, the fact that Christ bore the punishment due to sinners, the necessity of faith in Christ as Savior and Lord. He can be told that he CANNOT save himself through good works.
But ask that same man why God should let him into heaven...and you consistently get the same response: I hope I'm good enough, I try to do the right thing.
They heard nothing. They understood nothing. It made no sense. They are, as the Scripture's said, blind. All they got from the sermon was the need for making moral improvements to their life. They missed the gospel. They missed grace. They missed Christ. They missed heaven.
I have had these conversations frequently with church-goers over the years who hope they are saved. I had one just the other day. It borders on frightening. So many times they have heard the gospel. So many times they have missed it. While it confirms the truth of Scripture, it breaks my heart at the same time. It is possibly the most convincing, yet disturbing, proof of Christianity that I know.