The reference of St. Augustine adds to the the credibility of Dr. King’s argument about just and unjust laws mainly by

I think it adds to King's credibility by using the words of Catholicism’s most famous apologist St. Augustine. The quote enables King to reach an audience far beyond American blacks.

Augustine’s proclamation, that “charity is no substitute for justice withheld,” helped form Catholic moral law and inspired the young Baptist preacher to pursue his ministry with a particular eye on the public sphere. In his Letter from a Birmingham Jail, Dr. King writes, "I would agree with Saint Augustine, that an unjust law is no law at all."