"St. Luke says, speaking of the birth of our Saviour, that Mary brought forth her first-born son. Therefore, says a certain writer, if the evangelist affirms that Mary brought forth her first-born, is it to be supposed that she afterwards had other children? But the same author adds; If it is of faith that Mary had no other children according to the flesh except Jesus, then she must have other spiritual children, and these we are.
Our Lord revealed this to St. Gertrude, who,reading one day the passage of the Gospel just quoted, was troubled, not knowing how to understand it, that Mary being mother of Jesus Christ alone, it could be said that he was her first-born. And God explained it to her, by telling her that Jesus was her first-born according to the flesh, but men were her second-born according to the spirit". - The Glories of Mary