From the Bl Cardinal Schuster's Liber Sacramentorum
The Roman feast of September 29 included, indeed, together with St Michael, the whole of the heavenly armies, and the Collect of that Mass declares that the special Office entrusted by God to the Angels is that of watching faithfully over us. This profound teaching of the Doctors of the Church establishes a wonderful unity between the visible and the invisible world, and between mortal and angelic beings. The higher orders of creatures are in close communion with the lower orders, whom they enlighten and protect. . . . .
. . . Paul V in a decree of September 27, 1608, declared the feast of the Guardian Angels duplex ad libitum, and ordered that it should be kept on the first vacant date after the feast of St Michael. Clement X assigned it, later, to October 2.
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The guardianship of the faithful is given by God into the hands of the angels, not only for the sake of the unity and harmony which bind together the orders of creation, but also on account of the duty owed by the angels to Christ. It is from Christ that they receive their glory, therefore out of gratitude to him who is caput hominum et angelorum these blessed spirits watch over the Church, the chosen Bride of the Saviour, and over the faithful who are the members of his mystical body.
The epistle [for this feast] is taken from Exodus xxiii, 20-23, in which the Lord promises to the people of Israel journeying towards Palestine the guidance and help of his angel.
The angel represents God himself; he is overshadowed by his ineffable name, wherefore the children of Israel cannot offend him with impunity. He is also the avenger of the outraged sanctity of God, and has power to punish that carnal people and bring them to obedience and fear of him.
In many stories of the saints we notice the same severity shown towards them by their guardian angels, who punished the slightest imperfection of which those chosen souls were guilty.
The motive of this loving discipline may be sought, perhaps, not only the wise counsels of Providence aiming at the purification of certain predestined souls who were inspired to attain extraordinary sanctity through the ministry of the angels, but also in the exquisite perfection of the angelic nature itself, which is less able to understand human nature than are Jesus Christ and his Blessed Mother, having no personal experience of human weakness. "Non habemus Pontificem qui non possit compati infirmitatibus nostris, probatus per omnia, absque peccato."
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The Gospel is that of May 8. The Saviour's words clearly reveal the wonderful mystery of his mercy, which forms the object of to-day's festival. Every Christian, no matter how small or humble he may be, has an Angel to guard and watch over him.
This great truth should fill us with love and respect for our neighbours, and more especially for the weak and for the little ones, who cannot rely upon their own powers and influence. Let us be careful not to grieve our neighbour, lest the anger of his angel by aroused against us.
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We should have a great veneration for the holy angels, to whom God has assigned the care and government of creation. St Paul had so great a feeling of reverence for the angels, that when he directed that Christian women should cover their heads in church, as a sign of modesty, he desired that this should be done propter angelos, that is to say out of respect for the blessed spirits.
An old Anglo-Saxon prayer to the guardian angel first recorded by a monk of Hyde Abbey, Winchester some time in the mid 11th century, translated and published by Abbot Gasquet in his "Ancestral Prayers"
I believe, O holy Angel, that thou art appointed by God Almighty to be my guardian, Wherefore through Him who has set thee to this task, I, who am so miserable, so weak, and so unworthy, humbly ask that thou wilt ever and always in this life watch over me; that thou wilt guard me from all evils and ever shield me. And when God shall bid my spirit depart hence, let not the devils have power over it, but do thou gently receive my soul from my body and lovingly bear it into Abraham's bosom, at the biddng and by the help of my Creator, my Saviour, and my God, who is blessed for ever and ever. Amen.