Question 110 of the Small Catechism answers, “It means three in one. The Church has used the word Trinity to maintain the Bible’s witness that the Father, Son, and Holy Spirit are three distinct persons and yet are one God. This is the greatest mystery of the Christian faith.”
What is a mystery? Contrary to how it’s often used, a mystery is not the same as a secret. A secret is something hidden and unknown; a mystery is something revealed and therefore known, yet unfathomable. Thus, the Trinity is not a secret since in it, God has revealed Himself to be three distinct Persons yet one God. Yet His self-revelation is unfathomable, which is what makes it a mystery. When we speak of the Trinity, “I may not say that there are three gods as there are three men or three angels. Rather, I must say that there is only one eternal God. To be sure, a threeness does exist in the Godhead, but this threeness exists in the Persons of the Godhead. Not three Gods, not three Lords, not three Creators, but one God, one Lord, one Creator, or, as we are want to say: One divine Essence and yet three distinct Persons—Father, Son, and Holy Ghost” (What Luther Says §4450).
Anti-trinitarians like to retort that this doesn’t make mathematical sense since 1 + 1 + 1 = 3. I’ve heard Trinitarians rebut, “Yes, but 1 x 1 x 1 = 1.” I’m not certain this is helpful since it is still an attempt to use human reason to understand what it cannot comprehend, and it likely undermines the distinctions of the Persons. After all, it is not a mathematical question but an ontological one. Rather, a far simpler confession when asked how it’s possible that God is three distinct Persons in one Being would be, “I don’t know. I simply take God at His Word.” For that is faith, which is “the assurance of things hoped for, the conviction of things not seen” (Hebrews 11:1). Faith does not cling to human reason but to the proclamation of God’s Word, whether those words be, “I and the Father are one” (John 10:30), “This is My body, this is My blood, given and shed for you for the forgiveness of sins” (Matthew 26:26-28), or “Behold, I am coming soon” (Revelation 22:12, 20). “Unlike reason, faith does not depend upon what it sees, but grasps what it does not see… Faith closes its eyes to what reason would look at and judge” (Baseley, 39).
Baseley, Joel. Christ Beyond Reason: Luther’s Treatment of Faith and Reason in the Festival Portions of the Church Postils. Dearborn, MI: Mark V Publications, 2005.