According to the Bible, the Creator of the universe is an infinite and morally perfect being. However, the same scriptural text states that human beings are (by nature) creatures, which means they are finite and, because of sin, imperfect. Yet, for the creature to encounter the Creator, the creature must first acknowledge his or her limitations, boundaries, and imperfections. Thus, from a biblical perspective, human beings must become acquainted with mystery if they are to come to know the infinite, eternal, and unchanging God. For as evangelical theologian Bruce Milne states in Know The Truth, p. 52:
Mystery inevitably accompanies all God’s dealings with his creatures.
The doctrine of the Trinity, while understandable as a teaching, is shrouded in divine mystery. For the one God subsists eternally and simultaneously as three distinct, but not separate, persons. Some people reject the Trinity doctrine because it doesn’t comport with their experience or because they cannot fully comprehend it. However, the question is whether human beings will accept God as He reveals Himself to be, mystery included, or only settle for a being they think they can fully comprehend.
Of course if the human mind could comprehend God, could he be much of a God? According to historic Christianity, the truths of God may range above reason, but never against it. The Christian faith rejects both pure rationalism (all truths can be discovered through human reason) and irrationalism (truth is unreasonable). Thus, the Trinity is mysterious, but not contradictory.
For more on the historic Christian doctrine of the Trinity, see “How Can God Be Three and One?” in my book Without a Doubt: Answering the 20 Toughest Faith Questions.