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.
God the Father is He that has always been, is, and always will be. God the Father created all that existed, including time. Time is not infinite. There will come a day when time will end. God's love is infinite just as He is. The Nicene Creed says "We believe in God the Almighty, creator of Heaven and earth, of all that is seen and unseen". The Spanish translation is "of all that is visible and invisible."
God the Son is the perfect reflection of God the Father. The Son accepts the infinite love of the Father and returns the same to Him. The Creed says "We believe in One Lord Jesus Christ, the only son of God, ETERNALLY (just as God the Father is eternal) begotten of the Father. God FROM God, Light from light, TRUE GOD FROM TRUE GOD......Begotten, NOT MADE, one IN BEING with the Father".
Now, in the same way that when a man and wife fully give their love to each other, and the result of that love is a new person; the complete giving and receiving of love between God the Father and God the Son is the Holy Spirit. The Creed says "We believe in the Holy Spirit, the Lord, the Giver of life. WHO PROCEEDS FROM THE FATHER AND THE SON. TOGETHER WITH THE FATHER AND SON THEY ARE WORSHIPPED AND GLORIFIED.
Still just one God, but 3 components if you will. All resulting from the infinite love of God the Father.
Of course the Council of Nicea was a policy meeting, for lack of a better word, of theologic scholars and Church Officials. Is there any evidence of the Trinity in Scripture?
Of course in the New Testament, Jesus teaches us to pray: "Our Father, who art in Heaven". And he tells his Apostles that they will recieve the Holy Spirit, which they do in the form of tongues of fire. And Jesus refers to himself often as the Son of Man. When he ascended into Heaven, he told he would be returning to his Father. Which the Creed says "He ascended into Heaven, and is seated at the right hand of the Father".
Most Christians, and All Jews have read the Book of Genesis, Being one of the 5 Books of the Pentateuch that makes up the Jewish Torah. The first chapter of Genesis contains 2 descriptions of Creation. In verses
1-25 (NIV Bible, though I use the RSV) describes the creation of the Earth and the Heavens. Verse 26 reads that God says "Let us make man in OUR IMAGE, in OUR LIKENESS, etc. This is scriptural proof of the existance of the Trinity. If there is one God and no Trinity as Jews and Muslims believe, Why does God say "OUR"? This is a Trinitarian statement from God's Holy Word.
During the Creation description, When God created things, He saw that they were GOOD. In Genesis Chapter 2, God forms a Man from dust, and breathes into his nostrils the breath of life. In verse 7, God places the Man in the Garden he has created in Eden. Verse 18 shows us another Trinitarian statement. God says "It is NOT GOOD for the Man to be alone". Why did he decide so? Because HE is not alone, the Son and the Holy Spirit are with him.
None of us can ever fully understand the Trinity, as our minds are but a speck of dust to the knowlege of God. But at least we now have proof that this mystery of our faith actually exists.