10 Starting from this point you may understand the majesty of the Holy Ghost also. For the Gospel witnesses of Him that when the angel said to the Virgin, “You shall bring forth a Son and shall call His name Jesus, for He shall save His people from their sins,” she replied, “How shall this be, seeing I know not a man?” on which the angel said to her, “The Holy Ghost shall come upon you, and the power of the Highest shall overshadow you. Wherefore that holy Thing which shall be born of You shall be called the Son of God.” See here the Trinity mutually cooperating with each other.
The Holy Ghost is spoken of as coming upon the Virgin, and the Power of the Highest as overshadowing her. What is the Power of the Highest but Christ Himself, Who is the Power of God and the Wisdom of God? Whose is this Power? The Power of the Highest. There is here then the Highest, there is also the Power of the Highest, there is also the Holy Ghost. This is the Trinity, everywhere latent, and everywhere apparent, distinct in names and persons, but inseparable in the substance of the Godhead. And although the Son alone is born of the Virgin, yet there is present also the Highest, there is present also the Holy Ghost, that both the conception and the bringing forth of the Virgin may be sanctified.
Source: Commentary on the Apostles' Creed (New Advent)