Jesus doesn’t just speak the truth: he is the truth.
Because of this, our core beliefs aren’t things we get to pick and choose based on personal preferences; instead, they’re defined and determined by what Jesus says to us in his word of truth, the Bible.
There’s a lot we could say about that, but the central truths the Bible teaches can be summarised like this: that there is one true God—the Father, the Son and the Holy Spirit—who is the Creator of all things. God has entered history in his Son, Jesus Christ, who was born of a virgin, lived a perfect life, died for our sins, was buried, and on the third day rose again from the dead. One day, he will return to judge the world with perfect justice, remove all evil and all evildoers from his creation, and so make all things new. He has given his Holy Spirit to his people, the Church, so that we can know him, and love and serve him in his world. Faith in Christ alone is the only way to forgiveness of sins, rescue from his coming judgment, and life forever in his perfect kingdom.
For a fuller summary of the Christian good news, see What Do Christians Believe?
Following the sixteenth century reformation, the Bible’s teaching was summarised in more detail in documents like the 39 Articles and Westminster Confession of Faith in England, and the Heidelberg Catechism on the Continent. More recent summaries can be found in the UCCF Doctrinal Basis and the Crosslinks Declaration of Belief.