Tax collector's prayer

Luke 18:13

"God, be merciful to me, a sinner."

A priest once taught me this very simple and short Act of Contrition: "Lord Jesus, have mercy on me, a sinner."  Though every priest in the confessional has accepted it, and it suits my poor memory very well, I've always wondered how something so short can be substituted for the long and seemingly more complete Act of Contrition.

Now I have my answer.

In Luke, Jesus tells the parable of the Pharisee and the Tax Collector praying in the temple.  It is the Tax Collector, saying "God be merciful to me, a sinner" with true contrition who goes home justified.