Credo.  The Apostles’ Creed is placed at the beginning of Matins, because Matins is the beginning of the whole Office, and faith is the beginning, the principium of every supernatural work.  St. Paul teaches us that it is necessary for us to stir up our faith when we approach God, “For he that cometh to God must believe that He is.”  In reciting the Creed we should think of the sublime truths of our faith, and our hearts should feel, what our lips say, “For with the heart we believe unto justice; but with the mouth confession is made unto salvation” (Rom. x. 10).  We should remember too, that this formula of faith comes to us from Apostolic times and that it has been repeated millions of times by saints and martyrs; their sentiments of belief, of confidence in God and love of God should be ours.

Domine labia mea aperies.  The practice of this beautiful invocation dates from the time of St. Benedict (480-553).  In his Office it stood after the words Deus in adjutorium.  These words Domine labia mea aperies, taken from the Psalm Miserere, remind us of God purifying the lips of Isaias His prophet with a burning coal, of how God opened the lips of Zachary to bless God and to prophesy.  “And immediately his mouth was opened and his tongue loosed, and he spoke blessing God” (St. Luke, i. 64).  Very appropriately, does the priest reciting the Divine Office ask God to open his lips, to fortify his conscience, to touch his heart.

Deus in adjutorium.  These words, the opening words of Psalm 69, were always and everywhere used by the monks of old, says Cassian, who called this short prayer the formula of piety, the continual prayer.  The Church repeats it often in her Office.  St. John Climacus says it is the great cry of petition for help to triumph over our invisible enemy, who wishes to distract us and to mar our prayer.  It should be said with humility and with confidence in God.  In repeating these holy words we make the sign of the Cross; for, all grace comes from the sacrifice of the Cross; and besides, it is a holy and an ancient practice to begin all good works with the sacred sign.

Gloria Patri.  This little prayer indicates the purpose and end of the recitation of the Office, the glory of the Holy Trinity.  “Bring to the Lord glory and honour; bring to the Lord glory to His name” (Psalm 28).  The many repetitions of this formula in the Church liturgy shows the great honour which she pays to it, and the trust she places in its efficacy.  It was especially loved by St. Francis of Assisi, who said that it contained all wisdom.

This form of doxology, “Glory be to the Father, to the Son, and to the Holy Ghost,” was adopted to repel Arianism, by giving to the faithful a compact theological formula by which they could end every dispute.  Some authors quote St. Ephrem (circa 363) as the originator of this much-used prayer.  The form would seem to be of Syrian origin, translated into Greek and later into Latin (Dom Cambrol, Dictionnaire d’ Archeologie Chretienne, I., 2282, et seq., word Antienne, Liturgie; Month, May, 1910).