The Jehovah's Witnesses are well trained in deflecting common arguments trying to disprove their theology, including those on their rejection of the Trinity. Their arguments are generally based on what they've been taught by the Watchtower, which in the case of the Trinity is derived by misquoting Church Fathers and Scripture twisting. Yet the Witnesses can sometimes be reached with the truth by combining kindness with a proper Scriptural presentation.
In the following lecture from several decades ago, Dr. Walter Martin respectfully explains the Watchtower position on the Trinity and then relates it to the doctrine of the Trinity as found in the Bible. Since this recording was made many years ago, some of the information contained in the lecture has been superceded by changing Watchtower doctrine, but the Trinitarian truths found in Scripture do not change. Dr. Martin established the Christian Research Institute in 1960, which is still with us today; it publishes the apologetics magazine Christian Research Journal. He also authored the still-in-print and updated book The Kingdom of the Cults.
Listen in as Dr. Martin delivers a great apologetics lecture on the Trinity: