‘Jesus presents this latter petition as the sine qua non of the believer in his parable about the Pharisee and the tax collector in Luke 18. It was the tax collector who went home justified because he humbled himself, beating his breast and crying out, “God, be merciful to me a sinner!”

Not exactly: in verse 9 Luke says that Jesus told the parable “to some who trusted in themselves that they were righteous, and treated others with contempt”. Jesus summary statement in verse 14 is “I tell you, this man went down to his house justified, rather than the other. For everyone who exalts himself will be humbled, but the one who humbles himself will be exalted.” The parable is as much about not being justified as it is about being justified. The pharisee went home not justified, not because he failed to ask for mercy, but because he exalted himself and treated others, the tax collector in particular, with contempt. One might say, taking the parable in a Lutheran direction, that if the pharisee had been humble he would have recognized his need for mercy and asked for it. That, in my view, misses the crucial point. The sine qua non is genuine humility, before God and particularly before other people, not a petition or a troubled conscience.