 In 1634, at the king's order, he engaged in a polemic with the Jesuits, who endeavored to defend the conversion of Christian William, Margrave of Brandenburg to Roman Catholicism.Against this pamphlet Brochmand delivered a series of lectures which, after his death, were collected and published under the title "Apologiæ, speculi veritatis confutatio" (1653).His reputation as a dogmatist was established by his "Systema universae theologiae" (2 vols., 1633) in which he proved himself a firm opponent, not only of the Roman Catholics, but also of Calvinism.