In the introduction to his  unparalleled work, The Holy Trinity: In Scripture, History, Theology and Worship, Robert Letham includes a quote from an email correspondence with Sinclair Ferguson. In a straightforward fashion, Ferguson defended the absolute necessity of preaching the doctrine of the Trinity from Christ's teaching in the Upper Room when he wrote:

“I’ve often reflected on the rather obvious thought that when his disciples were about to have the world collapse in on them, our Lord spent so much time in the Upper Room speaking to them about the mystery of the Trinity. If anything could underline the necessity of Trinitarianism for practical Christianity, that must surely be it!" (Letham, The Holy Trinity, 375)