Many people like to give a philosophically argued answer to this question. For such a response, please see the links on the left titled “Arguments for God” and “Arguments for Christ.”
But human arguments aren’t the best way to know the Bible is true. God’s activity in our life is the best way to know Him, by which I mean God’s personal self-disclosure in nature and through the testimony of the Holy Spirit.
In Romans 1:18-22 we read:
18 For the wrath of God is revealed from heaven against all ungodliness and unrighteousness of men, who by their unrighteousness suppress the truth.
19 ¶ For what can be known about God is plain to them, because God has shown it to them.
20 For his invisible attributes, namely, his eternal power and divine nature, have been clearly perceived, ever since the creation of the world, in the things that have been made. So they are without excuse.
21 For although they knew God, they did not honor him as God or give thanks to him, but they became futile in their thinking, and their foolish hearts were darkened.
22 Claiming to be wise, they became fools,
So God makes Himself known to all men through nature.
Concerning those who hear the word of God, Christ told His disciples in John 16:7-11:
7 ¶ “Nevertheless I tell you the truth. It is to your advantage that I go away; for if I do not go away, the Helper will not come to you; but if I depart, I will send Him to you.
8 “And when He has come, He will convict the world of sin, and of righteousness, and of judgment:
9 “of sin, because they do not believe in Me;
10 “of righteousness, because I go to My Father and you see Me no more;
11 “of judgment, because the ruler of this world is judged.
And the Bible further says in 1John 2:22:
22 Who is a liar but he who denies that Jesus is the Christ? He is antichrist who denies the Father and the Son.
This means the Holy Spirit’s testimony regarding Jesus Christ is so clear that we are called liars if we deny it. A person is not called a liar because they misunderstand something, because they don’t apprehend something, or because they make a mistake. A person is only called a liar when they say something they know is not the truth. So the Holy Spirit’s testimony is such that it makes us know the truth about Jesus.
This is the most powerful way of knowing the Bible is true.
But what about those who don’t hear about Jesus before they die?
2 Peter 3:9 says:
9 ¶ The Lord is not slack concerning His promise, as some count slackness, but is longsuffering toward us, not willing that any should perish but that all should come to repentance.
In Romans 9:15 we read:
15 For He says to Moses, “I will have mercy on whomever I will have mercy, and I will have compassion on whomever I will have compassion.”
And Acts 17:26,27 says:
26 “And He has made from one blood every nation of men to dwell on all the face of the earth, and has determined their preappointed times and the boundaries of their dwellings,
27 “so that they should seek the Lord, in the hope that they might grope for Him and find Him, though He is not far from each one of us;
Therefore, all whom the Lord chooses to have mercy on for His reasons, God causes to be born at a time and place where they will hear the gospel.