Luke 18: 9-14 To some who were confident of their own righteousness and looked down upon everybody else, Jesus told this parable: “Two men went up to the temple to pray, one a Pharisee and the other a tax collector. The Pharisee stood up and prayed about himself: ‘God, I thank you that I am not like other men- robbers, evildoers, adulterers- or even like this tax collector. I fast twice a week and give a tenth of all I get.’ “But the tax collector stood at a distance. He would not even look up to heaven, but beat his breast and said, ‘God, have mercy on me, a sinner.’ “I tell you that this man, rather than the other, went home justified before God. For everyone who exalts himself will be humbled, and he who humbles himself will be exalted.”
Cast The First Stone 
Great post, Julia. We at times tend to forget that no one is perfect, and that all praise belongs to Him. When one achieves, one needs to recognize that God is the one who made this possible. In all things, give thanks to Him. Blessings.
This Lucan passage is described as a Parable and it is, however, what seems to be the dominant meaning, would have been understood by most now and when Christ walked the earth.
A parable is defined as earthly story with an intentionally hidden heavenly meaning. Luke 18.9-14 it is one of the few parables which has an easily understood “earthly meaning” yet, its true meaning was hidden at the time.
Jesus clarifies this in the parable of the wheat and the Tares Mt. 13.36-43.
“And then the disciples came, and asked Jesus, Why dost Thou speak to many in parables? 11 He answered: Because the knowledge is given unto you to know the mysteries of the kingdom of heaven, but to them this knowledge is not given.”
He then goes on to describe true meaning, since the Tares parable has no evident earthly meaning. Mt. 13.36-43.
The parable from Luke, however, is a very select and unique parable.
“God be merciful to me a sinner”
This man considered himself to be τῷ ἁμαρτωλῷ To hamartolon, literally (The sinner of sinners). There is no indirect article in Koine Greek. However, I can understand translators rendering the passage with the indirect article “a” for context which is done often, but which does not exist in Greek. Another rather complex way to render what this man was asking God for is to say:
“God, justify me the sinner of sinners upon the basis of an expiatory sacrifice which satisfies the demands of divine justice and makes possible the just bestowal of righteousness on the basis of justice satisfied.”
Wuest, K. S. (1961). The New Testament: An expanded translation (Lk 18:9–14). Grand Rapids, MI: Eerdmans.
This man’s prayer is a humble call for forgiveness, however, the word used for merciful here in the Koine Greek (ἱλάσκομαι, Jilaskomai) is a request for a propitiation (a permanant everylasting substitutionary atonement), which is associated with the concept of a request for atonement now and forever. (c.f. Ps 51:1, 3; 25:11; 34:6, 18).
Jesus replied:
“I tell you that this man, rather than the other, went home justified before God.
I’ll leave out the Greek and wordiness :-) and simply say that the word Justified (a verb) in this case is used in the as a perfect, passive, participle. Very rare in the NT and the hidden meaning of this parable was/is that in both the Old and New Testaments, it has always has been the faith of a repentant and humble sinner that pronounced a sentence of Justified, now and forever.