From The Good News We Almost Forgot by Kevin DeYoung:
Augustine once commented about the Trinity that "in no other subject is error more dangerous, or inquiry more laborious, or the discovery of truth more profitable." More recently, Sinclair Ferguson has 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!"
One of the many reasons the Trinity matters is because the love and relationship we find in Christian community flows out of the constant and eternal relationship the Father, Son and Holy Spirit experience; the perfect love shared by the Trinity can be modeled and experienced by those in fellowship with the Father, Son and Holy Spirit.