Every time we receive the sacraments, every time we make the sign of the cross and pray, every time we show Christian love to our neighbour, we are acknowledging the involvement of the Triune God in our lives. The Trinity is at the centre of our Christian life: it is the mystery of God who eternally knows and loves himself and invites us to share in His knowing and loving. This doctrine represents the culmination of Divine revelation, and so it is very fitting that the Solemnity of the Trinity occurs at this point in the liturgical year. The Incarnation, Christ's Passion and Resurrection, the Ascension and Pentecost, is the story of what the Trinity looks like projected onto our broken world. When we see the Cross we see how God understands himself, and how he communicates this understanding to all humanity.
The doctrine of the Trinity could be summed up by saying that the one true God exists as three persons each of the same Divine nature: the Father is God, Jesus Christ is God, the Holy Spirit is God, each of them is distinct, yet God is indivisible. This is not a riddle, but a gift. God created the universe freely out of pure love, but God's loving does not depend on the universe. It is not as though he had to create the universe so that He had something to love. God is Trinity so God has always loved; His love is eternal. The doctrine of the Trinity is a gift because it means that God is revealing the knowledge of His eternal love to us. He does not keep us at a distance, but he lets us actively participate in showing His eternal love to the world - and this is what it means to be holy.