The much maligned doctrine of the Trinity is an assertion that, appearances to the contrary notwithstanding, there is only one God.
Father, Son, and Holy Spirit mean that the mystery beyond us, the mystery among us, and the mystery within us are all the same mystery. Thus the Trinity is a way of saying something about us and the way we experience God.
The Trinity is also a way of saying something about God and the way he is within himself, i.e., God does not need the Creation in order to have something to love because within himself love happens. In other words, the love God is is love not as a noun but as a verb. This verb is reflexive as well as transitive.
If the idea of God as both Three and One seems far-fetched and obfuscating, look in the mirror someday.
There is (a) the interior life known only to yourself and those you choose to communicate it to (the Father). There is (b) the visible face which in some measure reflects that inner life (the Son). And there is (c) the invisible power you have in order to communicate that interior life in such a way that others do not merely know about it, but know it in the sense of its becoming part of who they are (the Holy Spirit). Yet what you are looking at in the mirror is clearly and indivisible the one and only You.
From “Listening to Your Life: Daily Meditations with Frederick Buechner” by Frederick Buechner