This dissertation is a theological comparison on the themes of justification and union with Christ in the thought of Bernard of Clairvaux (1153) and Martin Luther (1546). Primarily, the sources considered are Bernard's Sermons on the Song of Songs: Sermones super Cantica Canticorum (1135-1153) and Luther's 1535 Galatians Commentary: In epistolam S. Pauli ad Calatas Commentarius, for these biblical exegetical sources enable comparison for the way in which these thinkers intend justification and union with Christ to be understood pastorally. Various emphases or trends in scholarship on these two thinkers are considered: the emphasis in Luther scholarship on an external notion of justification away from notions of union or indwelling, and an emphasis in Bernadine scholarship away from Cross-centred notions of justification and towards ones of union, likeness, or ascent. The outcome of the dissertation challenges such trends. For it finds - in addition to notions of union, likeness, or ascent - a clear theology of justification through faith in Christ in Bernard's teaching, with Christ's death on the Cross standing as a key element within this, and here language of unlikeness is also used. It finds both themes of union and indwelling - in addition to forensic and external notions of justification - present and central to Luther's thinking even in his later thought at a time when his Reformation theology had become more sharply defined.