22 This faith, and every part of it, is impressed upon us by the evidence of the Gospels, by the teaching of the Apostles, by the futility of the treacherous attacks which heretics make on every side. The foundation stands firm and unshaken in face of winds and rains and torrents; storms cannot overthrow it, nor dripping waters hollow it, nor floods sweep it away. Its excellence is proved by the failure of countless assaults to impair it. Certain remedies are so compounded as to be of value not merely against some single disease but against all; they are of universal efficacy.
So it is with the Catholic faith. It is not a medicine for some special malady, but for every ill; virulence cannot master, nor numbers defeat, nor complexity baffle it. One and unchanging it faces and conquers all its foes. Marvellous it is that one form of words should contain a remedy for every disease, a statement of truth to confront every contrivance of falsehood. Let heresy muster its forces and every sect come forth to battle. Let our answer to their challenge be that there is One Unbegotten God the Father, and One Only-begotten Son of God, perfect Offspring of perfect Parent; that the Sun was begotten by no lessening of the Father or subtraction from His Substance, but that He Who possesses all things begot an all-possessing Son; a Son not emanating nor proceeding from the Father, but compact of, and inherent in, the whole Divinity, of Him Who wherever He is present is present eternally; One free from time, unlimited in duration, since by Him all things were made, and, indeed, He could not be confined within a limit created by Himself.
Such is the Catholic and Apostolic Faith which the Gospel has taught us and we avow.
Source: On the Trinity (New Advent)