How do we know what to believe in? If you ask the average person why they believe the way they do, they'll probably tell you they reasoned it out on their own. The problem with human reasoning is that it has never been proven to be infallible. Our intellect is a gift from God but if it is not being lead by the Holy Spirit, it is like an unguided missile out of control, "There is a way that seems right to a man, but its end is the way of death" (Proverbs 14:12).
Each of us are defined by our body, soul, and spirit (1 Thessalonians 5:23). Our bodies are our temporary, physical habitation in this world. The soul is what defines our personality, where our thoughts, memories, and emotions manifest themselves. The human spirit is that portion of ourselves that has an awareness of God and communes with Him if we are spiritually alive.
All people are born in this world spiritually dead. A person's spirit must be born from above, or "born again" in order to enjoy a relationship with God (John 3:3). If a person knows that Jesus was God Himself who paid the penalty for the sins of those who believe that He was raised from the dead, their spirits have been born again (Romans 10:9).
Being born-again does not mean we are instantly perfect. We are forgiven and God sees us in our future sanctified condition after we are resurrected and when we will be perfect. Until then, we still inhabit a body and soul which is in a world cursed with sin. There will be struggle with sin because we still have our old sin nature alongside our born-again spirit (Galatians 5:17). The difference is that we no longer have to be controlled by that old sin nature because we now have the Holy Spirit to guide us (Ephesians 4:22-24).
God does indeed reveal spiritual truths to us via His Holy Spirit to our born-again spirit (1 Corinthians 2:9-10). These truths manifest themselves as thoughts. Those dependent only on their human spirit can not truly understand spiritual matters even though many pretend to. "The natural man does not receive the things of the Spirit of God, for they are foolishness to him; nor can he know them, because they are spiritually discerned" (1 Corinthians 2:14).
Do you see the potential problem with relying on your intellect or your feelings? How do we know our brilliant thoughts are of God or our old sin nature? This is why God has given us His Word. If our bright ideas are of God, they will be consistent with the Bible. If they conflict, they can not be from God.
The believers in Berea were praised for not being satisfied with the Apostle Paul's word as truth by itself but searched the Scriptures to verify that his teaching was indeed from God (Acts 17:11).
So how about one's own experiences as a source of our beliefs? If you saw or felt something, it was real wasn't it? Here again, there is nothing wrong with basing beliefs on personal experiences so long as they are consistent with the Bible.
The Apostle Peter was an eye witness to a miracle where Jesus was glorified on the mount of transfiguration per Matthew 17:1-5. Yet he did not base his belief that Jesus was the Son of God on this experience but saw it as a reinforcement of God's Word. Seeing Jesus fulfill prophecies about the Messiah is what convinced Peter that Jesus was the Son of God (2 Peter 1:19-21). Incidentally, the term "Son of God" means same as God (see John 10:33 in context).
Just because we have a supernatural experience, it does not mean it is of God. Jesus warned that there would be "great signs and wonders to deceive, if possible, even the elect" (Matthew 24:24).
The other source of authority we have to be careful about is tradition. This is particularly prevalent in religious circles. The Gospels are full of stories about encounters Jesus had with the Jewish religious leaders over their traditions. The Scribes and Pharisees came up with rules that were their interpretation of Scripture called the "Talmud" then held them up as equal to Scripture. These rules were more about enslaving people than leading them to the grace of God as was the intention of God's Law.
Traditions in the church can be good if they lead people to Christ. But if they become empty rituals, they are worthless. As a Christian, you should conduct your life, "knowing that you were not redeemed with corruptible things, like silver or gold, from your aimless conduct received by tradition from your fathers, but with the precious blood of Christ, as of a lamb without blemish and without spot" (1 Peter 1:18-19).
So, we find that of the four basic sources of authority we have upon which to base our beliefs, only Scripture is trustworthy! "Beware lest anyone cheat you through philosophy and empty deceit, according to the tradition of men, according to the basic principles of the world, and not according to Christ" (Colossians 2:8). Since Scripture was inspired by God (2 Timothy 3:16-17), it is consistent with itself and can be used as the scales by which we can weigh all ideas that come to us from other sources. If we think we find an inconsistency in the Bible, there is something to be learned because the inconsistency must be a result of our not understanding something. We can not accept the inerrancy of Scripture if we do not know the nature of God; if we know God, we know He is Almighty enough to have accurately presented His Word to mankind and preserved it through history even with His error-prone creation.
The purpose of the Bible is to lead people into an eternal relationship with God through Jesus Christ. Those who sincerely seek to know God will know the truth. Jesus said, "I am the way, the truth, and the life" (John 14:6). Spiritual truth is more important than anything you can see, have, or experience in this present world. This world is temporary and will pass away but God's Word endures forever (1 Peter 24-25).