How the Bible Works.
The Bible isn't like a dictionary or the internet. We can't go to it and look up a topic that interests us and read how the Bible defines it. We can't Google Jesus's baptism. The Bible is more like a story book in which many smaller stories fit together to make up one big story (the story of how God saves His people and all His creation).
The many people who were inspired by God to write the Bible knew that people learn more from stories than from abstract definitions. We may not listen to someone who tells us, "Don't abuse other people's trust!" But we will listen to the story of a shepherd boy who liked to get attention by running into his village shouting, "There's a wolf attacking the sheep!" and who, when a wolf really did attack his sheep one day, found out too late that nobody would believe him anymore. The story teaches us something we need to know, so effectively that all we have to do is to say that someone is "crying wolf" for anyone to know right away what we mean.
This is what the Bible does. It teaches the truth about God by telling us stories that educate us. The story of the Good Samaritan teaches us that the person we most hate is actually our neighbor, and we have to look out for them as well as we do ourselves. The story of Jesus on the Cross teaches us (with many other things!) that nothing, not even death, can stand in the way of God's love for us. So to understand "what the Bible says," we have to learn "what the story means."
What's the Story?
Let's look at the story of Jesus's baptism. Here's how the Gospel of Matthew tells it:
"In those days came John the Baptist, preaching in the wilderness of Judea, "Repent, for the kingdom of heaven is at hand." For this is he who was spoken of by the prophet Isaiah when he said, "The voice of one crying in the wilderness: Prepare the way of the Lord, make his paths straight." ... Then Jesus came from Galilee to the Jordan to John, to be baptized by him. John would have prevented him, saying, "I need to be baptized by you, and do you come to me?" But Jesus answered him, "Let it be so now; for thus it is fitting for us to fulfil all righteousness." Then he consented. And when Jesus was baptized, he went up immediately from the water, and behold, the heavens were opened and he saw the Spirit of God descending like a dove, and alighting on him; and lo, a voice from heaven, saying, "This is my beloved Son, with whom I am well pleased." (Matthew 3:1-2, 13-17).
We notice right away that St. Matthew quotes the book of Isaiah from the Old Testament. This happens a lot in the New Testament. Why? Because the New Testament writers had to convince anyone who doubted that Jesus was the Messiah by proving that the word of God itself (which in those days meant only the Old Testament) said that he was. So St. Matthew makes his audience remember a famous passage from Isaiah which speaks of the coming of the Messiah. By reminding them of a story they all know, he not only suggests that Jesus ("the Lord" who is on "the way") is the Messiah, but that Isaiah's story is true because someone (John) has appeared in the wilderness of Judea to proclaim Jesus's coming.
Remember how we only need to say that someone is "crying wolf" for everyone to remember the story of that boy? St. Matthew uses the same approach here. He doesn't mean for his audience (which knew the Old Testament very well) to think just of one verse of one of Isaiah's prophecies, but of all the verses of the prophecy. When we read the New Testament, we understand it much better if, every time we see a quotation from the Old Testament, we find that quotation and read the entire passage from which it comes.
Here's a famous example. Just before Jesus dies on the Cross, he cries out, "My God, my God, why have you forsaken me?" If we don't recognize this as the first verse from Psalm 22, we might psychologize it to mean that Jesus is simply expressing the depth of his suffering. But psychology didn't exist in Biblical times, and when we read the rest of Psalm 22, we find out that while the writer of the psalm begins by expressing despair, he ends by praising God's power to raise up the afflicted. The Gospel writer isn't just telling us about Jesus's suffering. He is urging those who believe in Jesus to look beyond the Cross and know that God has not actually abandoned Jesus, but will raise him up.
Now we know how to crack the code of the story of Jesus's baptism. The chief clue to the story's meaning is the Jordan river. This takes the careful reader back to chapter 3 of the book of Joshua in which, under Joshua's leadership, the people of Israel cross the Jordan into the promised land. The waters of the river (like the waters of the Red Sea in the book of Exodus) stand for the ancient forces of chaos and death. These would overwhelm the people if they were not protected by the word of God, the stone tablets in the Ark of the Covenant which is held up by the priests in the midst of the river so that the people may pass safely on dry ground (Joshua 3).
In the same way, the story of Jesus's baptism tells how Jesus, the Word of God made flesh, leads the way through chaos and death (the Cross) to bring his people to the promised land, the Kingdom of God which he preaches in the following chapters of Matthew. Jesus doesn't need to be baptized for his own benefit (as John the Baptist himself points out). But his baptism shows us that Jesus is a new Joshua (both names are from the Hebrew y'shua, "God saves") who is not merely a man, but also the Word of God who leads and protects his followers as they pass through death into the new promised land, the Kingdom of God.
The feast of Jesus's baptism is usually called Epiphany (among Western Christians) or Theophany (among Eastern Christians). Both words mean that Jesus is revealed as divine, not just because the voice from heaven says so, but because of what Jesus does. The voice from heaven and the appearence of the Holy Spirit confim that he is truly the Son of God after he is baptized, not before. The story shows us that it is through His Word that "God saves." It also shows us that by hearing and keeping that Word, we too may be saved, though we pass through death itself.