The idea of free trade in Grotius's Mare liberum and his legal opinion De iure praedae has a strong theological basis. Grotius called the right to travel and trade freely a ius sanctissimum, a 'sacrosanct law'. He also perceived the Freedom of the Seas as being a direct result of the will of God. This theological background was strategically necessary because Grotius developed the Mare liberum and the De iure praedae to argue against Spanish-Portuguese claims to a trade monopoly that also had theological underpinnings. But the theological aspect of Grotius's theory was also emphasized by the references he made to the Dominican friar Francisco de Vitoria's ius communicationis. This precursor to Grotius's Freedom of the Seas, which Vitoria had developed in his De indis, is connected to the legal justification of Christian mission and so has a clear theological connotation. In Grotius's work, Vitoria's concept of a universal right to Christian mission supervised by the pope was transformed into a theologically supported right to free trade. With this transformation of the ius communicationis into the principle of the Mare liberum, Grotius develops a theological basis not for politics but for economics. One can speak therefore, following Giorgio Agamben, of an 'economic theology' in regard to Grotius, a term that is, in turn, derived from Carl Schmitt's notion of 'political theology'.