Evangelists have often urged lost men and women to pray this "sinner's prayer" if they desired to be saved. The account does say that this publican, after praying thus, "went down to his house justified" (v. 14).
But there is more here than appears on the surface. It is not merely God's mercy that is needed for He has already been merciful to let us continue to live at all. The word translated "merciful" is used only one other time in the New Testament and is there translated "make reconciliation for." Speaking of the saving work of Christ, it says that He came "to make reconciliation for the sins of the people" (Hebrews 2:17). It is also closely related to the words for "propitiation" and "mercy seat."
This parable of the Pharisee and the publican is set in the context of the Jewish temple worship, where sinners would bring their sacrificial offerings to cover their sins, knowing that "it is the blood that maketh an atonement for the soul" (Leviticus 17:11). Such sacrifices were completely worthless, however, if offered in a spirit of religious pride and/or self-righteousness, like those of the Pharisee. There must be repentance and faith in God's promise of forgiveness through the death of an innocent substitute, pre-figuring the true Lamb of God whose coming death would truly make eternal reconciliation for the sins of the people. The publican prayed in this vein, and he was saved.
In our day, on the other side of the cross, a sinner's saving prayer must say, in effect: "God, be propitiated to me on the basis of the death of Christ for my sins." Such a prayer, offered in sincere repentance and faith in God's promise, brings justification before God. HMM