Simeon is used to bring great comfort to Mary and to Joseph. ‘And Simeon blessed them’ – he spoke well about them; that is what the word ‘blessed’ means.
The last phrase, ‘that the thoughts of many hearts may be revealed’, relates back to Christ being set for the fall and rising of many. Our response to the gospel, our response to Christ, reveals what is in our hearts. It reveals it for those who have eyes to see. Many like to hide their motives, their attitudes, their innermost thoughts, but the gospel draws them out into the open. Because all that God has to say to us is summed up in Christ, then our attitude to him exposes our entire attitude to God. He is the test case that proves whether or not we love God. Do we love his beloved Son who he sent into the world to teach us about him and to reconcile us to him?