The Unity of God is necessarily implied in the order of Nature, in the Faith, and in Baptism. The gifts of the Magi declare (1) the Unity of the Godhead; (2) Christ's Godhead and Manhood. The truth of the doctrine of the Trinity in Unity is shown in the Angel walking in the midst of the furnace with Shadrach, Meshach, and Abednego.
31. All nature testifies to the Unity of God, inasmuch as the universe is one. The Faith declares that there is one God, seeing that there is one belief in both the Old and the New Testament. That there is one Spirit, all holy, grace witnesses, because there is one Baptism, in the Name of the Trinity. The prophets proclaim, the apostles hear, the voice of one God. In one God did the Magi believe, and they brought, in adoration, gold, frankincense, and myrrh to Christ's cradle, confessing, by the gift of gold, His Royalty, and with the incense worshipping Him as God. For gold is the sign of kingdom, incense of God, myrrh of burial.
32. What, then, was the meaning of the mystic offerings in the lowly cattle-stalls, save that we should discern in Christ the difference between the Godhead and the flesh? He is seen as man, He is adored as Lord. He lies in swaddling-clothes, but shines amid the stars; the cradle shows His birth, the stars His dominion; it is the flesh that is wrapped in clothes, the Godhead that receives the ministry of angels. Thus the dignity of His natural majesty is not lost, and His true assumption of the flesh is proved.
33. This is our Faith. Thus did God will that He should be known by all, thus believed the three children, and felt not the fire into the midst whereof they were cast, which destroyed and burnt up unbelievers, while it fell harmless as dew upon the faithful, for whom the flames kindled by others became cold, seeing that the torment had justly lost its power in conflict with faith. For with them there was One in the form of an angel, comforting them, to the end that in the number of the Trinity one Supreme Power might be praised. God was praised, the Son of God was seen in God's angel, holy and spiritual grace spoke in the children.
Source: On the Christian Faith (De fide) (New Advent)