If Christianity is true, then why doesn't everybody believe it? I think the reason is because we can't really know anything about God, and one person's ideas about God are just as good as someone else's.
If we truly couldn’t know anything at all about God, then you’d be right: one person’s opinion would be just as good (or bad) as that of someone else. We’d be like those of whom Job spoke: “They grope in darkness with no light” (Job 12:25).
But we can know about God—and not only know about Him, but come to know Him as our personal friend! The reason is because God has not left us to wander around in a fog, never able to see clearly who He is or how He wants us to live. Instead, God has taken the initiative to reveal Himself to us—and that makes all the difference. God has not remained hidden, but He’s done everything possible to make Himself known to us.
How has He done this? First, He has revealed Himself through the world He created. We are not here by chance, and neither is the world around us. God created everything—and His fingerprints (so to speak) are all around us. As the Bible says, “God’s invisible qualities—his eternal power and divine nature—have been clearly seen” (Romans 1:20).
But God has revealed Himself most of all by becoming a man: Jesus Christ. Do you want to know what God is like? Look at Jesus, for He was God in human flesh. And when you do, you’ll not only realise that God exists, but that He loves you and wants to welcome you into His family forever. Doubt gives way to truth when Christ comes into our lives.