But since he says, in addition to this, “What is this preference of sinners over others?” and makes other remarks of a similar nature, we have to reply that absolutely a sinner is not preferred before one who is not a sinner; but that sometimes a sinner, who has become conscious of his own sin, and for that reason comes to repentance, being humbled on account of his sins, is preferred before one who is accounted a lesser sinner, but who does not consider himself one, but exalts himself on the ground of certain good qualities which he thinks he possesses, and is greatly elated on their account.
And this is manifest to those who are willing to peruse the Gospels in a spirit of fairness, by the parable of the publican, who said, “Be merciful to me a sinner,” and of the Pharisee who boasted with a certain wicked self-conceit in the words, “I thank You that I am not as other men are, extortioners, unjust, adulterers, or even as this publican.” For Jesus subjoins to his narrative of them both the words: “This man went down to his house justified rather than the other: for every one that exalts himself shall be abased; and he that humbles himself shall be exalted.” We utter no blasphemy, then, against God, neither are we guilty of falsehood, when we teach that every man, whoever he may be, is conscious of human infirmity in comparison with the greatness of God, and that we must ever ask from Him, who alone is able to supply our deficiencies, what is wanting to our (mortal) nature.
Source: Against Celsus (New Advent)