There is more than one Mystery of Catholic Faith. One is "Christ has died, Christ is Risen, Christ will Come again". We say or sing this at Mass when the Priest sings "LET US PROCLAIM THE MYSTERY OF FAITH".
And indeed there are other mysteries of our Catholic Faith. The mystery of the Holy Trinity, of the "three persons in one God". This Mystery is especially difficult for us as humans to understand. One of my uncles refused to convert to Catholicism because no Priest could convince him of the existance of the Trinity, or how God consists of 3 persons.
Catholicism varies from other monotheistic religions like Judaism and Islam in that we believe in the Trinity. If you will, Catholicism is "Trinitarian Monotheism".
Let us examine the 3 persons of the Trinity, with some help from the Nicene Creed and the Book of Genesis.