Manuscript 15631 from Spain's Biblioteca Nacional El nacimiento de San Juan Bautista. Auto Sacramental has received scarce critical attention. This has encouraged us to look into the unknown occurrences at Seville's Cabildo Municipal in 1610, the date in which the play is said to have been first performed. From within the text we have tried to unearth clues to establish its authorship (Diego de Villegas? Messía de la Cerda? Other dramatists?), while attempting to clarify eventual connections between the different people who signed the manuscript. This paper also includes the semi-paleographic and critical edition of this eucharistic play