This lecture by James White is based on his book, The Forgotten Trinity. He discusses the three foundational truths or teachings of the doctrine of the Trinity, that if you eliminate just one of them, you go off into some heretical error. Learn these foundations! Then he spends some time explaining how to explain or defend the Trinity when talking with adherents of other religions (e.g. Mormons, Jehovah’s Witnesses). This is a really important and educational lecture. Also, if you don’t already have his book, then I highly recommend it. The Trinity is a central doctrine of the Christian faith, yet far too few Christians understand its Scriptural basis and can defend it. You may also check out a blog post I did on the Trinity a while back: Trinity: A Practical Overview & Encouragement for Further Study.