Whenever the humility of the Virgin Mary is talked about, the obvious reference would be the episode of the Annunciation. In that event, an angel visited with the news that she will bore the Son of the Most High God and that the Holy Spirit will come upon her and overshadow her (Luke 1:35). Mary, which I think still had a lot of questions in her heart, answered with her “fiat”.
Mary's “fiat” is the word that shows her humility and utter dependence to God. Thus, many will call her the humblest of all of God's creatures.
One time, as I pondered the Third Glorious Mystery of the Holy Rosary, I was struck with a thought about the humility of our Mother. Mary, I mused, once again let the Holy Spirit overshadow her during the Pentecost.
The death of our Lord shook all of His disciples. Many hid in the upper room where Jesus took His last meal. They locked the doors in fear of their persecutors. The appearance of Jesus after His resurrection stirred hope into the hearts of His disciples but it is still not enough. They are still afraid.
The Apostles turned to Mary to seek the comfort of her faith in God. She acted as their mother in a way that Jesus told her while He is hanging on the cross. Up to the day of Pentecost, all of Jesus followers look up to the Virgin Mary.
If Mary is a lesser being then she can easily claim the role as the leader of the early Church. No one can contest her, even Peter, because she can easily say to apostles these words:
“You Peter have not been faithful up to end. You betrayed my Son three times and you fled. All of you, His apostles left him. I have been faithful to Him to the end. I stayed with Him at the foot of the cross. He gave all of you to me as your mother, higher than each and every one of you.”
Our Mother, however, is not a lesser being. She is the woman who is pure and pleasing to God. Thus, when the Holy Spirit came upon them at Pentecost she once again let the power of God overpower her. She who only desires to please God pleased Him once again because she desired His will alone. She desired Peter to be the leader of the Church together with the other Apostles.
Jesus said that those who humble themselves will be exalted in Heaven. Mary is the humblest of all thus God exalted her to the pinnacle of His creations. He assumed her to Heaven (4th Glorious Mystery) and crowned her (5th Glorious Mystery) to become the Queen of all Saints.