In general, the greatest blessing of Christmas, from a religious standpoint, is the birth of Jesus—and the hope and love associated with it. In the Christian faith, there is one God, and Jesus Christ (His son) is the long-promised Messiah—the one who would save God's people from their sin.
In terms of the blessings of Christmas, Jesus is God's promise of a Savior. Ultimately, Jesus is crucified to save people from death through the forgiveness of sins: achieved by dying on the cross. This demonstrates God's love for humanity.
Therefore, since we have been justified by faith, we have peace with God through our Lord Jesus Christ. Through him we have also obtained access by faith into this grace in which we stand, and we rejoice in hope of the glory of God. (Romans 5:1-2)
In terms of the greatest blessing of Christmas, it is hope: the hope of forgiveness through Christ's sacrifice on the cross, and God's love in giving His son to die.
Secondary Christmas blessings come in many forms: first, in having enough. Like Thanksgiving, people often feel blessed that family is able to come together for the holidays and that there is enough food. There is joy in fellowship that is shared on Christmas Eve and/or Christmas Day, and that members of the family and even the extended family are healthy enough to be together. Somehow, though the world is much the same on December 21 as on the 25th (for example), goodwill abounds on those days where people feel kinder, more generous and more loving. There is also the blessing of giving.
In Act 20:35, Paul repeats what Christ himself once said:
It is more blessed to give than to receive.
This theme can be seen in two ways. First there is the sense that a generous spirit is a blessing to others. There is something noble and charitable to give without thought of receiving something in return. Second, however, is a deep truth about the concept of giving: when done with a generous heart, the giver is blessed beyond one's understanding as he/she experiences the pleasure of sharing with others who have less: less material wealth or less spiritual or emotional comfort in the world. For the giving is not necessarily about the "thing," but often about the emotional place from where one gives: the selflessness that accompanies the gift to let someone know, "I care about you."
It is important to note that these "secondary" blessings are those experienced by Christians and non-Christians alike; for some people, Christmas is about Santa Claus and is more secular in nature. However, the blessings seem to be the same, except that Christians generally thank God for these blessings, whereas others don't necessarily consider God a part of Christmas.
Christmas blessings are hope, fellowship, generosity, love, and giving. These blessings are found in Biblical scriptures, as well in the world at large—as Christmas brings out the best, it seems, in an inordinate number of people, often with a desire to help others.
One need only look to Ebenezer Scrooge to understand the "Christmas spirit" and the wish to share one's blessings. Scrooge feels blessed to be alive with a second chance at helping others:
I’ll raise your salary, and endeavour to assist your struggling family...Bob Cratchit!
[Scrooge] became as good a friend, as good a master, and as good a man, as the good old city knew...and it was always said of him, that he knew how to keep Christmas well...