“But I cannot understand the doctrine of the Trinity,” says someone, “and because I cannot understand it, I don’t believe it.”
Stop for a moment and think about what you are trying to understand. God in three persons: Father, Son, Holy Spirit. That is the message of the Bible. Not “the ground of all being,” not “the ultimate,” not “the absolute,” not some vague spirit of goodwill, not “a spirit of love”— but persons who think, who act, who intervene, who do things. God the Father, creating at the beginning: “In the beginning God created.” God is not a force that brought things into being but a person who decided to create, who did so, and who, having created, goes on sustaining. God the Son, active, appearing in various angelic forms in the Old Testament, and, when the fullness of the time was come, born as a babe in a stable in Bethlehem (see Galatians 4:4). The Son is a living person. “He that hath seen me,” He said, “hath seen the Father” (John 14:9). “I and my Father are one” (John 10:30) Here also in Acts is this great emphasis upon the Holy Spirit…
Oh, I am not saying that I understand the Trinity, and I am not asking you to understand it. I am simply telling you that you will go to a Christless eternity unless you believe this message of the God who is and always was, the three persons in this blessed Godhead—coequal, coeternal in every respect, God acting in this world of time. It is not our world; we did not make it; we did not bring ourselves into it. We are in the hands of the living God. He is the author of everything, the sustainer of everything, and we live our lives under Him—these measured little lives that we have in this world as we pass through it. Oh, the idiotic conceit of men and women. They cannot understand themselves, nor God, nor anything without this view of spiritual reality.
David Martyn Lloyd-Jones, Courageous Christianity (1st U.S. ed.; Wheaton, Illinois, Crossway Books, 2001), p. 242.