This sermon is all about trusting in God's justice. Not thinking we can do God's job of judging fairly. Not letting our preoccupation with the unfairness we see all round damage the way we live out our Christian life.
But it doesn't mean we sit back an ignore injustice; we mustn't get chewed up by it. It ends with the sober reminder that God will judge us all when we leave this earth. And we should remember not only the times when we are on the end of injustice, but those occasions when we perpetrate injustice.
We'll need God's grace to save us for the punishment we deserve. And the only way to do that is to accept Jesus' sacrifice on the Cross as payment for our sins. And to thank God for his love that made all this happen.