The world may have been a very different place had the 1541 Colloquy at Regensburg reunified the church. That effort nearly succeeded when agreement was reached on justification, but heated discussion on church authority and the Mass left the church more deeply divided than ever. Nearly five centuries later, strikingly similar language appeared in a modern statement on justification. The modern agreement, hailed as a milestone of ecumenism for Lutherans and Roman Catholics, shows strong parallels with the fifth article on justification in the Regensburg Book. A clear understanding of the successes and failures of the discussions at Regensburg may provide insight into the modern pursuit of church unity. This monograph is a study of the context and documents of the 1541 Colloquy at Regensburg, including an original translation of the 23 articles of the Regensburg Book. Intended for students and scholars of the Reformation, those interested in modern ecumenism may also find this work informative and accessible.