The purpose of the parable we read today in the Gospel of the Mass is to distinguish authentic piety from false piety. True prayer goes through the clouds of heaven, as we read in the First Reading, always goes up to God and comes down full of fruits.
Before narrating the parable, St. Luke is careful to point out that Jesus was speaking to some who trusted in themselves and considered themselves righteous and despised others. The Lord speaks of two characters well known to all the listeners: Two men went up to the Temple to pray: one was a Pharisee, the other a tax collector. We immediately notice that, although the two men went to the Temple for the same purpose, one of them did not pray. He does not speak with God in a loving dialogue, but with himself. There is no love in his prayer, and no humility. The Pharisee stands, gives thanks for what he does, is satisfied. He compares himself with others and considers himself to be more just, a better fulfiller of the Law. He does not seem to need God.
The publican "stood afar off, and therefore God came more easily to him. Not daring to lift up his eyes to heaven, he already had him who made the heavens.... Whether the Lord is far off or not depends on you. Love and he will come near. And he will be attentive, as no one has ever been, to everything we want to tell him. The tax collector conquered God with his humility and trust, because He resists the proud and gives His grace to the humble, and teaches us how our prayer should be: humble, attentive -with our mind fixed on the person to whom we are speaking-, trusting, trying not to be a monologue -like that of the Pharisee- in which we turn to ourselves, to the virtues we believe we possess....
In the parable beats the idea of humility as the foundation of our dealings with God. He wants us to go to prayer as poor children, always in need of his mercy. God," teaches St. Alphonsus Liguori, "likes you to deal familiarly with him. Discuss with him your affairs, your plans, your work, your fears and everything that concerns you. Do everything with trust and an open heart, because God is not accustomed to speak to a soul that does not speak to him. Let us flee in prayer from self-sufficiency, from complacency in the apparent or possible fruits in the apostolate, in our own ascetic struggle... and also from negative, pessimistic attitudes, which reflect a lack of confidence in the grace of God, and which are often manifestations of a hidden pride. Prayer is always a time of joy, trust and peace.