The Doctrine of the Trinity is important for many reasons; it helps define, for example, other crucial doctrines, such as Incarnation and Atonement. Christians are, technically speaking, not monotheists as much as they are Trinitarians.
I am amazed, nevertheless, at the number of pastors who never preach on the Trinity. I relate to Roger Olson's words in his wonderful 2002 book The Mosaic of Christian Belief (pages 133-134):
This is too bad; it's scandalous, in fact, given that our spiritual ancestors sometimes died for their devotion to the Holy Trinity.
...some Christians have become so exasperated by the seeming confusion surrounding belief in the Trinity...that they have functionally given up on it. They may be members of a church with the word 'Trinity' in its name; they may pay lip service to belief in something called Trinity if asked; they may sing a hymn about God's triunity now and then in worship. But fewer and fewer Christians seem actually to embrace the belief known throughout Christian history as 'Trinity'. As one modern Catholic thinker has said, modern Christians tend to be functionally unitarian.
One of the reasons this doctrine is so important is because of what it teaches us about God. From the Doctrine of the Trinity, we learn that God is social by his very nature; God exists in community.
We, therefore, as those devoted to the God who has revealed himself as triune, should also exist in community. We need the Church in order to faithfully incarnate Trinitarian doctrine...an important realization on the eve of Annual Conference.
But Sunday is not only Trinity Sunday, for in her wisdom, the Church has declared the day to also serve as Peace with Justice Sunday. Peace is more than simply the absence of conflict; it is a way of life in which each person has the essentials such as food, water, and shelter, in which sins such as sexism and racism don't exist, in which love is the crucial factor in human relations. In order to faithfully live in community, the Church must prayerfully support the elimination of poverty, war, racism, sexism, hate, and the like. True peace is intimately married to real justice.
And so, we are called to live as faithful Trinitarians, modeling the One God who exists in community, and doing the work of Christ by loving our sisters and brothers in the human family by prayer and sacrifice.
"The grace of the Lord Jesus Christ
and the love of God
and the communion of the Holy Spirit
be with you all."