"Blessed are those who mourn, for they will be comforted". Again as with the previous statement Jesus made about the poor in spirit, this one as well is rooted in the Old Testament. In the "Servant Songs" of the book of Isaiah, chapters 40-66, there is a sense of mourning. Among many of such statements there is chapter 61:2 "to proclaim the year of the Lord's favor, and the day of vengeance of our God; to comfort all who mourn". In these writings in Isaiah, Judah is lamenting captivity, a exile that has occurred into Babylon that was brought about by unfaithfulness to Yahweh. Statements such as this are made "For our transgressions before you are many, and our sins testify against us" (Isaiah 59:12). Yet in these writings as well a sense of hope emerges, a longing for Yahweh to act again as he did in earlier times in Hebrew history, the exodus from Egypt. There is indeed mourning, but God will act again in history, he will comfort them, they will again be led through the sea, again they will travel through the wilderness as it blossoms and blooms. Judah looks for another exodus, a greater one really that the "arm of the Lord" will accomplish. So as Jesus says "blessed are those who mourn" he isn't unveiling a new form of teaching per say, rather the "new" aspect of it is him saying this is now happening, "you will be comforted". As Jesus goes through Israel proclaiming the forgiveness of sins to people he is bringing comfort a reconciliation with God, a return from exile spiritually. As his cousin John the baptist is baptizing in the Jordan river, which is itself a type of passing through the sea (1 Corinthians 10:2), a greater exodus is beginning to happen.
What does this mean for me? What does mourning and being comforted from God look like today? It is still the same, we recognize our own sin or evil or imperfections or mess ups or whatever else you want to call it and we look to God to heal us from these, to comfort us with his love. I look at my own self and see a wretchedness about me that I loathe, I mourn over it. Yet as undeserving as I am God desires fellowship with me, God is interested in me, God wants to use me to work for him, God sent his Son to die for me. What comfort that is, what a new way of living that opens up. I was reading recently about the history of Christianity in Ireland, and St. Patrick of course was the man who brought the gospel to that country. He must have been a magnificent person, I look forward to meeting him, but in his own short autobiography his first statements about himself are "I, Patrick, a sinner, the rudest and least of all the faithful, and the most contemptible with the multitude". Kind of a surprising way for a man who has a holiday named for himself to begin his autobiography. I get a strong sense he knew what it was to mourn what he desired to change in his life. Paul himself called himself the "chief of sinners". But God has not abandoned us, he has brought comfort, Jesus himself experienced our "exile" on a Roman cross in order to bring us to God.