Now the birth of Jesus Christ happened this way: When his mother Mary was espoused to Joseph, before they came together, she was found to be with child of the Holy Spirit.
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Ambrose of Milan
De Spir. Sanct., ii, 5: That which is of any thing is either of the substance or the power of that thing; of the substance, as the Son who is of the Father; of the power, as all things are of God, even as Mary was with Child of the Holy Spirit.