E. Colden Ave. at S. Avalon Bl, Los Angeles - photo credit Camilo Jose Vergara
When Martin Luther King, Jr., confronted racism in the white church in the south, he did not call on Southern churches to become more secular. Read his sermons and "Letter from a Birmingham Jail." and see how he argued. He invoked God's moral law and the Scripture. He called white Christians to be more true to their own beliefs and to realize what the Bible really teaches.
He did not say "Truth is relative and everyone is free to determine what is right and wrong for them."
If everything is relative, there would have been no incentive for white people in the South to give up their power.
Rather, Dr. King invoked the prophet Amos, who said, "let justice roll down like waters, and righteousness as a mighty stream" (Amos 5:24).
The greatest champion of justice in our era knew the antidote to racism was not less Christianity, but a deeper and truer Christianity.
Tim Keller, The Reason for God (pp 64-65).