Tamar, the wife of Judah, is mentioned to remind Matthew’s readers that while Judah was chosen to be in the line of Christ (Matthew 1:2), that choice was not based on his worthiness. Judah was involved in sin with Tamar (Genesis 38:1–30). This shows that God’s election is based on his grace.
The first elaboration found in Matthew’s genealogy concerns Judah. Judah had many brothers, and he is merely one of the forefathers of the tribes of Israel. Yet God chose him specifically: Jacob the father of Judah and his brothers
(Matthew 1:2). This brief addition sheds light on God’s election of Judah and his brothers. Yet that which follows immediately thereafter shows that it was also the election of a sinful human being: Judah the father of Perez and Zerah by Tamar
(Matthew 1:3). Matthew purposefully mentions the black page in Judah’s life (Genesis 38:1–30), even while he skips over many other pages of Genesis. The focus here is not on Tamar as some kind of model for female depravity, however, but the emphasis rather falls on the fact that Judah’s election points to our need for grace.1
3 and Judah the father of Perez and Zerah by Tamar, and Perez the father of Hezron, and Hezron the father of Ram,