The fiat of the Blessed Mother, because it is living the will of God, is the way of union with God. It is the way of the saints and the mystics of the church, the way of holiness. Because of Mary's fiat, her complete surrender to become the Mother of God, we were given the power to become children of God.
Through the fiat, the soul gives the Holy Spirit permission to take possession of it. The Holy Spirit can then use us in any way He desires. Since God desires only what is best for us, it is fitting to give Him permission to reign over us and use us as His instruments. It is through our fiat that we become channels of grace to others, we become His victims, drawing souls to Him. The more our desire for union with Him grows the more we beg Him to purify us. We strive to do penance and to mortify ourselves so that we can become more useful instruments.
The fiat is the way to heaven. It is the way of surrender, of giving up ourselves to be lost in God, lost in God but truly found, because our true happiness is found only in living the will of God. Our Lady through her fiat teaches us that it is the way of faithfulness, humility, obedience and acceptance.
The mystical incarnation can only happen in us when we give our simple, humble, fiat. But when it is united to Mary's it has such power. It is going to Jesus through Mary. Through it we allow Him to be born in us. We are overshadowed with the Holy Spirit as Mary was. We give Him power over us. We allow ourselves to decrease so He can incease and it is then that He can manifest Himself in us so that when others see us, they no longer see us but they see Him who dwells in us.
The fiat of the saints was so complete that He could work miracles through them. They surrendered their intellect and they became doctors of the church. They gave Him their hands and He gave them the zeal to work tirelessly for His kingdom. They gave Him their voice and He filled them with the Holy Spirit and their words inflamed others. They surrendered their bodies and they became His martyrs.
Mary's fiat at the Incarnation began her spiritual motherhood. But it did not end there. There was more. At the foot of the cross she silently gives her final fiat, which was one of the greatest acts of love. It is beneath the cross that she accepts the motherhood of those who had just crucified Him, the motherhood of all of us. She is the greatest victim soul the world has ever known. She is our model who is always pointing to Him.
In praying the Chaplet of the Fiat, we honor Mary by meditating on her most profound response to the angel, "Behold the Handmaid of the Lord, be it done unto Me according to Thy Word." Meditating on these words, taking them into our minds, transferring them to our hearts and then putting them into action by surrendering ourselves to the will of God.