"If any doctrine makes Christianity Christian, then surely it is the doctrine of the Trinity. The three great ecumenical creeds - the Apostle's Creed, the Nicene Creed, and the Athanasian Creed - are all structured around our three-in-one God, underlying the essential importance of Trinitarian theology. Augustine once commented about the Trinity that 'in no other subject is error more dangerous, or inquiry more laborious, or the discovery of truth more profitable.'" - Kevin DeYoung, The Truth We Almost Forgot. 49.

Question 22 - What then must a Christian believe? 

Answer - Everything God promises us in the gospel. That gospel is summarized for us in the articles of our Christian faith - a creed beyond doubt, and confessed throughout the world. 

Question 23 - What are the articles?

Answer - I believe in God, the Father almighty, creator of heaven and earth. I believe in Jesus Christ, His only Son, our Lord, who was conceived by the Holy Spirit and born of the virgin Mary. He suffered under Pontius Pilate, was crucified, died, and was buried; He descended to hell. The third day He rose again from the dead. He ascended to heaven and is seated at the right hand of God the Father almighty. From there He will come to judge the living and the dead. I believein the Holy Spirit, the holy catholic church, the communion of saints, the forgiveness of sins, the resurrection of the body, and the life everlasting. Amen.

Question 24 - How are these articles divided?

Answer - Into three parts: God the Father and our creation; God the Son and our deliverance; God the Holy Spirit and our sanctification.

Question 25 - Since there is but One God, why do you speak of three: Father, Son, and Holy Spirit?

Answer - Because that is how God has revealed Himself in His Word: these three disctinct persons are one, true, eternal God.