Perhaps the most immediate and obvious association of angels with liturgy is the "preface" prayer of the Mass, which precedes the Eucharistic prayer. Many different prefaces mark the liturgical year, but they always mention the angels or the choirs of angels. Consider the following preface for the first Sunday of Advent:

Father, all-powerful and ever-living God, we do well always and everywhere to give you thanks through Jesus Christ our Lord. When he humbled himself to come among us as a man, he fulfilled the plan you formed long ago and opened for us the way to salvation.

Now we watch for the day, hoping that the salvation promised us will be ours when Christ our Lord will come again in his glory.And so, with all the choirs of angels in heaven we proclaim your glory and join in their unending hymn of praise:

Holy, holy, holy Lord, God of power and might, heaven and earth are full of your glory. Hosanna in the highest. Blessed is he who comes in the name of the Lord. Hosanna in the highest.

The Sanctus (Holy, Holy, Holy ...) repeats the words with which the angelic hosts worship God according to the vision seen by Isaiah in the Temple in Jerusalem: "I saw the Lord sitting upon a throne, high and lifted up; and his train filled the temple. Above him stood the seraphim; each had six wings: with two he covered his face, and with two he covered his feet, and with two he flew. And one called to another and said: "Holy, holy, holy is the LORD of hosts; the whole earth is full of his glory. And the foundations of the thresholds shook at the voice of him who called, and the house was filled with smoke" (Isaiah 6:1-4). St John reports a similar angelic hymn of glory and honor in the last book of the Bible (Revelation 4:8). During the Mass, then, the Catholic joins in the worship of the angelic choirs. In a vivid illustration of this point, the presbytery at the Cathedral Basilica in St Louis is surrounded by mosaics representing the nine choirs of angels, including the "Seraphim" representation on the right.  

Abraham's sacrifice of his son Isaac, characterized by unconditional faith and obedience, is the pattern for all worship in the Old Testament. Indeed, this sacrifice stands at the root of the whole Israelite cult that is centered on the Temple: Solomon built the Temple on the height in the land of Moriah, where God led Abraham to sacrifice Isaac. In the passage that follows, note the importance of the Angel of the LORD in Abraham's sacrifice:

"Then Abraham put forth his hand, and took the knife to slay his son. But the angel of the LORD called to him from heaven, and said, 'Abraham, Abraham!' And he said, 'Here am I.' He said, 'Do not lay your hand on the lad or do anything to him; for now I know that you fear God, seeing you have not withheld your son, your only son, from me.' And Abraham lifted up his eyes and looked, and behold, behind him was a ram, caught in a thicket by his horns; and Abraham went and took the ram, and offered it up as a burnt offering instead of his son...." (Genesis 22:10-13). This dramatic portrayal of the Sacrifice of Isaac was painted by Rembrandt van Rijn around the year 1650. The angel of the Lord intervenes in time to stop Abraham from applying the knife to Isaac.

For Christians, Abraham’s sacrifice is a type of the Eucharist, through which the Father (represented by Abraham) offers his Son (represented by Isaac, or by the lamb/ram). Angels also play a significant role in the sacrifice of Christ, the Lamb of God, who is worshipped both in heaven and on earth.

"...I saw a Lamb standing, as though it had been slain, with seven horns and with seven eyes, which are the seven spirits of God sent out into all the earth.... Then I looked, and I heard around the throne and the living creatures and the elders the voice of many angels, numbering myriads of myriads and thousands of thousands, saying with a loud voice, 'Worthy is the Lamb who was slain, to receive power and wealth and wisdom and might and honor and glory and blessing!' And I heard every creature in heaven and on earth and under the earth and in the sea, and all therein, saying, 'To him who sits upon the throne and to the Lamb be blessing and honor and glory and might for ever and ever!' And the four living creatures said, 'Amen!' and the elders fell down and worshiped." (Revelation 5:6, 11-14)

To the right is Jan van Eyck's artistic representation, completed in 1432, of the angels and saints adoring the Lamb in heaven. It is no coincidence that this work appears on the cover of the General Instruction of the Roman Missal, which describes and provides instructions for offering the Mass.

This image helps us to imagine the relation between the heavenly worship of Christ among the angels and saints and the eucharistic worship of his Church militant on earth. Eucharistic Prayer I includes the following related prayer: "Almighty God, we pray that your angel may take this sacrifice to your altar in heaven. Then, as we receive from this altar the sacred body and blood of your Son, let us be filled with every grace and blessing." The Mass lifts us to worship God with the angels and the saints in heaven.