No single phrase is repeated more often in divine worship the "Lord, have mercy" (Kyrie eleison in Greek, Gospodi pomiloi in Slavonic). For an explanation of these simple and essential words, we turn to St Nicholas Cabasilas' A Commentary on the Divine Liturgy:
The meaning of the prayer for God's mercy after every petitionSt Nicholas writes that "this prayer implies both gratitude and confession." It is noteworthy that the roots of our English word "mercy" likewise reflects both senses of the word: the Middle English comes from the French merci, which means both "pity" and "thanks."
There is another question to be asked: why is it that, whereas the priest asks them to pray for so many different things, the faithful in fact ask for one thing only – mercy? Why is this the sole cry they send forth to God?
In the first place, as we have already said, it is because this prayer implies both gratitude and confession. Secondly, to beg God's mercy is to ask for His kingdom, that kingdom which Christ promised to give to those who seek for it, assuring them that all things else of which they have need will be added unto them [Matt 6:33]. Because of this, this prayer is sufficient for the faithful, since its application is general.
How do we know that the kingdom of God is signified by His mercy? In this way: Christ, speaking of the reward of the merciful, and of the recompense of kindness which they will receive from Him, in one place says that they shall obtain mercy, and in another that they shall inherit the kingdom; thus proving that God's mercy and the inheritance of the kingdom are one and the same thing. "Blessed are the merciful, for they shall obtain mercy," He says [Mat 5:7]. And elsewhere, as if to explain Himself and to show what it means to obtain mercy, he declares: "Then shall the King say unto them on His right hand (by whom he means the merciful): Come, ye blessed of my Father, inherit the kingdom prepared for you from the foundation of the world" [Matt 25:34].
Moreover, if, among the actions of merciful men, one wishes to contemplate the aim of the divine mercy, he will find that it corresponds exactly to the kingdom itself. For what is the character of the merciful man? "I was hungered, and ye gave me meat; I was thirsty and ye gave me drink" [Matt 25:35]. Therefore those to whom Christ shows mercy He will admit to a share at His own table. And what table is this? "That ye may eat and drink at my table in My kingdom" [Lk 22:30]. And that you may know the splendour of that table, and may realize that it is a table not of servants but of kings, know that he who waits upon it is the Lord of all. "He shall gird Himself, and make them sit down to meat, and will come forth and serve them" [Lk 12:37].
Likewise, if I may recall that other saying: "I was naked and ye clothed me" [Matt 25:36], the Lord will clothe him to whom He shows mercy. And will He not give him a royal garment? He will clothe him in His own garments; and nothing of His is base, for He is a king – just as nothing of ours is royal, since we are slaves. This garment is the wedding-robe which will ensure for those who wear it admission to the kingdom; the King will find in its wearers no fault for which they can be cast forth from the marriage chamber.
What more? He will open His doors to them and lead them into His house to rest. "I was a stranger, and you took me in" [Matt 25:35]. Those who have been counted worthy of such favour are servants no longer, but sons of God. "The servant abideth not in the not in the house forever; but the Son abideth forever" [Jn 8:35]. Now the sons are heirs not only of the kingdom but of Him Who is the heart of the kingdom. "Heirs of God, co-heirs with Christ," says St Paul [Rom 8:17].
That is why, in asking Christ for mercy, we are asking that we may obtain the kingdom.
For further reflection on this most essential of prayers, see this essay.
The above icon depicts the parable of the wise and foolish virgins.