This is my favourite part of the song! The song shifts and sounds so hopeful and peaceful considering the reality of the world displayed in the preceding lyrics. It also feels so full of joy.
In Matthew 5:1-11, we read a list of sayings by Jesus known as "The Beatitudes" (which simply comes from the Latin for "happiness"). Jesus starts these eight sayings with "Blessed are those who . . . ." "Blessed" can also translated as "happy." Scholars, though, note that "blessed" is better because "happy" loses the religious significance.* Reading the whole passage, it's easy to assume the blessing is only for the second part of each Beatitude. For example, the blessing is in receiving the kingdom of God, being called a child of God, or inheriting the earth. I would say, though, the blessing (happiness), isn't just in the promise. The blessing is in both a person's current situation as well as God's future promise.
So, someone may be mourning, but God blesses even the mourning and promises comfort. You may be a peacemaker (at the cost of your own security), but in that, take joy and know that you are a child after God's own heart. We may be extending more grace and mercy than we think you have. Yet, we are blessed because there is more, not just for us to dispense, but for us to receive.
Joy (more than mere happiness) is this blessing. It is not bound by time. It is not bound by circumstance. It's not even bound by emotion. It stands ready to be embraced both in the darkness and in the light. I don't think joy promises an easy life. However, (speaking from experience) two choices are always before us. We can either let the bitterness and rage of today choke out hope for tomorrow. Or, we can hold on to the promise that we are blessed in the here and now. In addition to that, we can look forward to the goodness of God to come.
This is where Advent comes in. We wait. We anticipate. We prepare.
Our dreams of a better self, better relationships, a better understanding of calling, or a better world may not yet be realized. People bound by slavery, political wars, and systems may not yet be free from the oppression they face. Despite all this, as a person of faith, I live joy in seeking to bring heaven to earth today. At the same time, I cling to the joy that someday at Christmas time Christ will make all things new.
*Domeris, William. "'Blessed are you...' (Matthew 5:1-12)." Journal Of Theology For Southern Africa 73, (December 1990): 67-76. ATLA Religion Database with ATLASerials, EBSCOhost (accessed December 15, 2015).