While I am certainly not qualified to interpret the words of Pope Francis, I wonder if he is referring to the fact that we all are guilty of filtering the gospel a bit to fit our own perpective or ideology. Instead we all need to look at the Gospel message as a personal message to us for our own conversion - not the conversion of someone else.
Of course, that may or may not affect the leftists and convert them in any way. But, I do agree with him that social change takes place person by person.
I am very suspicious of anyone who tries to make a political point using biblical excerpts. All sides do it, and it's wrong. After decades of daily bible study, I am more convinced that Jesus didn't care a bit about political matters. He only ever focused on individual relationships with God and our fellow man.