Mary accepted the angelic message that she would give birth to a son conceived by the Holy Spirit. She did not choose this pregnancy but she accepted it. Her husband Joseph does not believe her story (do you blame him?). He wants to be faithful to the law and show compassion to his wife by divorcing her privately. He may have chosen Mary to be his wife but he did not choose this pregnancy. Yet, like Mary, Joseph accepted Jesus as his son despite this unexpected event.
The Holy Trinity is one of the greatest mysteries in the Bible. God, three-in-one is often difficult to explain and describe with our limited language.
God identifies with himself through love. You can say that the Spirit is what binds the love of the Father to the Son and the Son to the Father. This is the love that he pours out to his creation.
Love is what binds us to God, his mission, and to each other. The reason that the loving Father is so loving is because of the unique relationship that he has with himself. God knows what it truly means to love himself. This is why Jesus can say that the second greatest command is to love your neighbor as yourself.