He didn't say that. In that much misquoted letter which he sent to the atheist editor of La Reppublica, Pope Francis actually began that part of the conversation by saying "Premise that – and it’s the fundamental thing – the mercy of God has no limits if one turns to him with a sincere and contrite heart."
It's possible that part wasn't quoted in whatever garbled version of it you might have read. Tendentious accounts tend to cut it out; tendentious readers tend to pass over over it unseeingly, as if that weren't his premise; as if it weren't underlined as fundamental; as if he hadn't said it at all.
I accept the Pope’s premise, I reject his conclusion.
Care to post, in full context, whatever version you are referencing? Then we’ll be on the same page.