The mathematics of self-justification

Then Jesus told this story to some who had great confidence in their own righteousness and scorned everyone else: 
“Two men went to the Temple to pray. One was a Pharisee, and the other was a despised tax collector. The Pharisee stood by himself and prayed this prayer: ‘I thank you, God, that I am not a sinner like everyone else. For I don’t cheat, I don’t sin, and I don’t commit adultery. I’m certainly not like that tax collector!  I fast twice a week, and I give you a tenth of my income.’ 
“But the tax collector stood at a distance and dared not even lift his eyes to heaven as he prayed. Instead, he beat his chest in sorrow, saying, ‘O God, be merciful to me, for I am a sinner.’ 
Luke 18:9-14

What sorrow awaits you teachers of religious law and you Pharisees. Hypocrites! For you cross land and sea to make one convert, and then you turn that person into twice the child of hell you yourselves are! Matthew 23:15

I fast once per week which makes me half a Pharisee. And any who replicate my example, at least one times the child of hell I am if we think we are any better than anyone else.

In this instance, 1/2 a Pharisee is no better than a 1 whole Pharisee. The mistake is the same. Looking to justify myself.

'God be merciful to me a sinner.'

The prayer never changes. If anything the only change is the deepening truth of how much a sinner I am. And by extension how much greater his mercy must be!

I say it for me. I say it for all of us. Paul's admission:
This is a trustworthy saying, and everyone should accept it: “Christ Jesus came into the world to save sinners”—and I am the worst of them all. 
But God had mercy on me so that Christ Jesus could use me as a prime example of his great patience with even the worst sinners. Then others will realize that they, too, can believe in him and receive eternal life. 
All honour and glory to God forever and ever! He is the eternal King, the unseen one who never dies; he alone is God. Amen. 1 Timothy 1:15-17
isn't just one person's confession.
Paul lands on the birthplace of faith, mercy, and a mindset that disdains any form of self-justification.