6 This is the Holy Ghost, who came upon the Holy Virgin Mary; for since He who was conceived was Christ the Only-begotten, the power of the Highest overshadowed her, and the Holy Ghost came upon her, and sanctified her, that she might be able to receive Him, by whom all things were made. But I have no need of many words to teach you that generation was without defilement or taint, for you have learned it. It is Gabriel who says to her, I am the herald of what shall be done, but have no part in the work. Though an Archangel, I know my place; and though I joyfully bid you All hail, yet how you shall bring forth, is not of any grace of mine. The Holy Ghost shall come upon you, and the power of the Highest shall overshadow you; therefore also that Holy Thing which shall be born of you shall be called the Son of God.
Source: Catechetical Lectures (New Advent)