Question 104 in the Small Catechism succinctly answers, “The Creed summarizes all of God’s work in creation and human history as taught in the Bible.” Where did the Apostles’ Creed come from? It is so named not because the 12 Apostles wrote it (they didn’t), but it was developed from early inquiries of catechumens by the bishop. The earliest example of this appears around AD 215 from Hippolytus’ Apostolic Tradition. In short, the Creed is a summary of the Christian faith based on the foundation of the Apostles, Christ being the cornerstone (Ephesians 2:20). Though we ought to think of the Creed as a whole, we often describe it in three distinct articles according to each Person of the Holy Trinity (Father, Son, Holy Spirit). As Luther comments,
These distinctions are made that Christians may have the simple and certain assurance of the existence of only one God and yet three Persons in the one Divine Essence… This faith has come down to us as an inheritance, and until the present day God has maintained it with power in His church against all sects and devils. Therefore we must abide by it in simplicity and not be wise in our own conceit; for Christians are expected to believe things that seem foolish to reason.
What Luther Says §1044
The Creed is a brief summation of who God is: Father, Son, and Holy Spirit. He is our Creator (1st Article, the Father), our Redeemer (2nd Article, the Son), and our Sanctifier (3rd Article, the Holy Spirit). These are not “modes” in which He functions (the heresy modalism) but rather mysteriously three distinct Persons yet one Divine Being. Christians do not claim to fathom how this is possible; neither do we rely on analogies to explain this, for they all fall into some sort of anti-trinitarian heresy. Rather, we merely confess how God has revealed Himself, which is one God in three distinct Persons. For what reason? As the Small Catechism puts it, “That I may be His own.”