A study of J.S. Bach's Cantata 21, the fourth cantata composed by Bach in Weimar 1714 after his elevation to the position of concert master. Bach's longest cantata to this point in his career, Cantata 21 has in recent years suffered from adverse aesthetic judgements (especially its final chorus) as the result of lack of knowledge of its compositional history. Study of the cantata in relation to the relevant theological literature of the time reveals that it is a work of unusually careful construction. In the context of its companion works of 1714, Cantata 21 emerges as a pivotal work in Bach's oeuvre.