When I read this passage I am always reminded of the story of the publican and the sinner from Luke 18.
“Two men went up to the temple to pray, one a Pharisee and the other a tax collector. The Pharisee stood and prayed thus with himself, ‘God, I thank You that I am not like other men—extortioners, unjust, adulterers, or even as this tax collector. I fast twice a week; I give tithes of all that I possess.’ And the tax collector, standing afar off, would not so much as raise his eyes to heaven, but beat his breast, saying, ‘God, be merciful to me a sinner!’ I tell you, this man went down to his house justified rather than the other; for everyone who exalts himself will be humbled, and he who humbles himself will be exalted.”
A religion of works naturally lends itself to bragging. ‘Look how good I am. I am doing it all right. God must be happy with me.’
It was that kind of attitude that had the Jewish leaders where they were and why they as a rule rejected Christ. Paul had to dispel that teaching. Since salvation comes only by grace though faith there is no room to anyone to boast.
When we forget this, even today, we set ourselves up for a fall. Our free gift ought to humble us when we realise what Christ has done. Our prayer must always be ‘be merciful to me a sinner.’