In short the Pharisee imagines that he is everything and has everything, whilst he considers the publican to be nothing and to have nothing. What consummate wickedness! He should rather have said: It is true, I give my tithes regularly; I fast and do other good works as much as I can; yet I cannot trust in all my doing. All I have and do is a merciful gift of Thine, O God, and this publican may be more pleasing in Thy sight than I. Thus the Pharisee should have placed the publican above himself, or at least on an equality with himself. He should have said: It does not depend on gifts, either many or few, but on a gracious God. Why should I boast and despise others; when everything de pends on the mercy of God and not on the gifts which we may possess? But such is not his language. He rather boasts of his piety, and that at a time when he is standing before God and praying.
Thus the Lord forbids us to boast on account of our piety. He also teaches that no one has any reason to despair on account of sin, into which he has been misled by the devil. We have one God who embraces and covers us all with the mantle of His mercy — the pious and the sinner, the learned and the ignorant, the rich and the poor — He is the God of us all. Hence we should not exalt ourselves, but be humble, and should not imagine that we are better than others because we are rich, whilst they are poor. For God may favor the poor rather than the rich; yea, He can take away riches and reduce to nakedness, and again cover the naked beautifully and bestow excellent gifts. Why then should we exalt ourselves and despise others? (Luther’s House Postil, Sermon for the Eleventh Sunday after Trinity, Volume 3, p. 762)