We have been blessed immensely at Karis by the ministry of New York pastor Tim Keller.  Keller planted and serves Redeemer Presbyterian Church there.  An article we use and disperse much is his work called "The Centrality of the Gospel."  Keller argues that two "thieves" stand on opposite sides of our gospel, much as two men were crucified on each side of our Lord.  Those "thieves" are religion or legalism and irreligion or relativism.  In the paper, he states that both are equally opposed to the gospel of our Lord.  This important work profoundly taught me that legalism is as much of a problem as relativism-- something we don't often understand in evangelicalism. A number of Keller's works can be found on Steve McCoy's Reformissionary site here.  The "Centrality of the Gospel" piece can be found here.