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! (cited in Letham, The Holy Trinity, 375)
For an exegetical and theological exploration of this, see Father, Son and Holy Spirit: The Trinity and John's Gospel, by Andreas Köstenberger and Scott Swain (IVP, 2008), esp. 166-179.