Joseph is in a mess. You have to understand that what we call "engagement" was a much bigger deal in that culture than it is for us today. It was tantamount to marriage itself. (Notice that Joseph is referred to as Mary's "husband," even though they are only engaged.) The only difference between engagement or betrothal and marriage was that the engaged couple did not live together, and did not therefore consummate the marriage. If something went wrong between them during the engagement, they couldn't just drop the engagement and call off the wedding; the arrangement could only be ended with divorce.

Joseph is a righteous man, a deep-down good man. But he's confronted with a situation that's just about the worst thing for a righteous man: a mess in his own life and family. Righteous people don't like messes in their families any more than an accountant likes a spreadsheet that won't add up. Mary's pregnant, and as far as Joseph or anyone else is concerned, it's either him or another guy, and Joseph knows he's not the one. It's a mess.

What's the first thing this righteous man does? He decides to divorce Mary. Divorce is certainly not compulsory or inevitable in such situations, but, according to Jesus, it's a righteous action. The gospel upholds faithfulness as such a crucial aspect of marriage that its violation makes even divorce a permissible and righteous response. Joseph, who can see no other way, is a righteous man when he decides to divorce Mary.

This decision wasn't an easy one for Joseph. On the one hand he was "unwilling to expose [Mary] to public disgrace." On the other, he felt he needed to do the right thing, protect his good reputation, and divorce Mary. That's what any Rabbi would have advised him to do.

But Joseph's righteousness is a very special kind—it's merciful righteousness. Matthew clearly wants us to admire the kind of righteousness Joseph displays. Joseph could have vindicated his own moral honor, but he knew that divorce would expose Mary to deep shame and ostracism. Joseph cares too much for Mary to do that.

So he decides to "divorce her privately." Instead of going public with his grievance and getting his dowry money back, he could write a simple bill of divorce, which could be done for any reason at all with only two or three witnesses. Divorcing Mary this quiet way costs Joseph. He is no longer the righteous man who has been wronged by an adulterous wife. He can no longer claim the high ground of moral purity. By divorcing her quietly Joseph refuses to publicly deny he is the child's father, even though he knew he wasn't.

In the end, it takes another angel visit to straighten things out. The heavenly messenger tells Joseph the whole story and instructs him to call his son, Jeshua, Jesus, Savior, "for he will save his people from their sins." Then Matthew's gospel adds one more insight. It declares an Old Testament prophecy now fulfilled: "The virgin will conceive and give birth to a son, and they will; call him Immanuel, (which means, 'God with us')." God comes to be with us in our world, in our messes, in our sins. That's the promise of the gospel, and that's the joy of Christmas.