Within the Catholic Church the Sunday after Pentecost is known as Trinity Sunday (last Sunday). This day shines a light on one of Christianity's deepest beliefs: that God exists as Father, Son, and Holy Spirit. This is also known as the Trinity. I have taken the following extract from the Catholic Archidoecese of Melbourne to give more information about Trinity Sunday.
"The Christians who believed that Jesus was lesser than the Father then forced their opponents to address another central question. If Jesus, the Son of God, the Father and the Spirit are all God, how can they be the One God to whom Abraham and Jesus prayed, and not two or three gods. The Church called to resolve the disputes did not explain how God is three and one, but used the language of one God in three Persons to assert both that God is one being and that Father, Son and Spirit share fully in divinity. The prayers and sermons on the feast of the Trinity make that point again and again. It is not an explanation but a way of speaking that respects the full mystery of God’s love for us.
Unless we know a little about the history behind the words we use to describe God as Trinity, it is easy to dismiss the belief as concerned with divine mathematics. It really celebrates something much deeper: the depth of God’s involvement with us in the salvation that Jesus brought us. It insists that when Jesus spoke to us, God speaks, when Jesus dies on the cross, God dies in him, that when Jesus rises and ascends to God he takes us with him, and that when the Spirit works within our lives and Church, God works within us."