In a sense, every person is a theologian. Every person has thoughts about God. Each has an opinion about what God is like, that is, what his character and attributes are. Even an atheist is, in a sense, a theologian. Richard Dawkins, for example, is an atheist who spends a lot of time thinking, speaking , and writing about God. He does not believe God exists and is constantly trying to convince the rest of us.
Certainly every Christian must be a theologian. Although many Christians reject the necessity of knowing any theology in favor of “just loving Jesus”, it is a foolish concept. Why do you love Jesus? You love him because of what you think his character and works are. You love him because he loves you. You love him because he saved you. These are statements about Jesus’ character and works. They are theology.
You also know every Christian must be a theologian because of the existence of the Bible. Why did God go to the trouble of having his word written by dozens of prophets, kings, and apostles? Why did he preserve and protect it over the centuries? Obviously, God thinks his word is important. We think it is important because he does. It is important because it teaches us about him. It teaches us theology.
Moses wrote down the law and read it to the people before they crossed the Jordan and went into Canaan. He did this, at God’s command, so that the people would know who God is and who they were. They would know what God expected from them and what he would do for them.
God required the Israelite kings to hand write their own copy of the law. He did this so they would know God and his requirements for the nation.
Jesus taught his disciples. Some of his disciples wrote down what Jesus said and did. Those acts and deeds show us who Jesus is, what he did for us and why he did it. He told us to “abide in his word” so we would know these truths. (John 8:31) People who do not know these truths make up beliefs about Jesus, creating false Christs who are, in reality, idols.
As we study the Bible, and the theology it teaches, we learn more about God. As we learn more, we appreciate him more, increasing our love, worship and service.
We must also know theology to be a witness to others. Why should a person commit their lives to Jesus? It is because we are sinners under the wrath of God and subject to eternal punishment. It is because Jesus died to pay the penalty for our sins and give us eternal life. That is theology. If you do not know it, you cannot lead others to Christ for salvation. If you make up your own theology outside of the Bible, you become a false witness who leads people away from Christ and dooms them to judgment.