A Jehovah's Witness who had knocked on my door challenged me in the course of our conversation to give a clear and concise definition of the Trinity doctrine. Here's how I responded: The word "trinity" refers to "tri-unity" (three in one), thus conveying the biblically revealed truth that there is plurality within the unity of God's nature (one God in three "persons"). The doctrine of the Trinity should be understood within the broader context of the Christian view of God. The God unveiled in the Bible (and later described in the ancient creeds of Christendom) is the one sovereign and majestic Lord. Historic Christianity thus affirms belief in one infinitely perfect, eternal, and personal God—the transcendent Creator and sovereign Sustainer of the universe. This one God is triune. He exists eternally and simultaneously as three distinct (though not separate) persons: Father, Son, and Holy Spirit.
All three persons in the Godhead share equally and completely the one divine nature, and are therefore the same God—coequal in attributes, nature, and glory. God has revealed Himself as one in essence, but three in personhood. In terms of what God is (essence), God is one; in terms of who God is (personhood), God is three. God is therefore "one What" and "three Whos." The God of the Bible therefore reflects both a unity of nature (monotheism) and a plurality of personhood (trinitarianism).
Since the Trinity doctrine is so crucial for Christians, and since it is so often distorted or misunderstood by various critics, it is important for believers to be able to define this basic Christian doctrine. Even though the Trinity doctrine is not fully comprehensible to the finite human mind, what Christians believe about the doctrine is clear and distinct in the church's creeds and statements of faith. The truth of this doctrine, however, can only be clearly and cogently communicated if believers take seriously their responsibility to study and show themselves approved (2 Timothy 2:15).
For more on the doctrine of the Trinity, see Kenneth Richard Samples, Without a Doubt: Answering the 20 Toughest Faith Questions (Grand Rapids: Baker, 2004), chapter 5.