This page shows the Watchtower belief that one can believe something he cannot understand.  Does this mean also that one could believe in the trinity?

What does the Reasoning from the Scriptures book say?  This quote is taken from page 148.

"Did God have a beginning?

The Watchtower's New World "Translation",  Ps. 90:2: "Before the mountains themselves were born, or you proceeded to bring forth as with labor pains the earth and the productive land, even from time indefinite to time indefinite you are God."

Is that reasonable? Our minds cannot fully comprehend it. But that is not a sound reason for rejecting it..."

By the way, who can "partly" understand God's attribute of eternality?  Answer:  No one can!

Now using this same logic, let's apply it to the tri-unity (meaning one God who is triune) of God.  Because our minds cannot comprehend this, is that a sound reason to reject it?  The same answer applies.  NO!

Since God said He created man in His own image and likeness and man is a triune being, is it unreasonable to believe in a God who is also triune?  The same answer applies.  NO!

The tri-unity of man is clearly stated in scripture: 

1Thess 5:23  And the very God of peace sanctify you wholly; and I pray God your whole spirit and soul and body be preserved blameless unto the coming of our Lord Jesus Christ.

A person is one part material and two parts non material.  The material part of a person is the body.  It is that part of a person that is earth conscience.  Gen 2:7