Between the first edition of the Institutio Christianae Religionis in 1536 and the end of his stay in Strasbourg in 1541, Calvin's ecclesiological thought underwent a radical transformation. In 1536, Calvin still articulated an understanding of the Church that, while sharply critical of Roman abuses, sought to recover the original ancient catholic roots in an attempt to define a new identity, in search of a middle way that could simultaneously be authoritative and well-founded, in contrast both to the corruption of the Roman Church and to Anabaptist-leaning ecclesiologies. Calvin's exile to Strasbourg in 1538 profoundly altered this framework. Strasbourg confronted Calvin with the different urban, ecclesial, and political environment of a city that was being shaped decisively by the reforming activity of Martin Bucer. Bucer, who welcomed the exiled Calvin, arranged for him to serve as a pastor of the French congregation, appointed him as a lecturer in the Academy, and paved the way for him to meet Lutheran princes and theologians and to participate in the religious colloquies in Germany. From an ecclesiological perspective, this transitional period marked the development of a fully articulated "Reformed ecclesiology." The relationship between Calvin and Bucer has led some scholars to attribute his post-Strasbourg development largely to Bucer's influence. While acknowledging Bucer's shadow in the evolution of Calvin's ecclesiological thought, this paper aims at showing that Calvin's early ecclesiological architecture should also be understood as his own original response to the Roman Church, the growth of the French Reformation, and the alarming spread of Anabaptist ecclesiologies. Against this backdrop, his epistles and his work on the Commentary on Romans are crucial elements to distinguishing what is uniquely 'Calvinian' in his ecclesiology and what is more indebted to other sources, including Bucer, identifying its precise contours and manifestations.