In the eighteenth century, Newton's ideas were an important element of the dominant world view, but it was not immediately clear that they could play such a role. What made them seem philosophically relevant is investigated in the case of the Dutch Republic. At the universities people initially were rather sceptical about Newton's theories. Support came mainly from scientific amateurs who tried to place their religious feelings on a scientific footing. The Spinozists' claim that religion was refuted by mathematics created the need for a scientific answer, which would demonstrate the fallaciousness of Spinoza's geometrical method and uphold revealed religion. In this article, it is shown in some detail how Newtonian ideas were put to such use among mathematical amateurs in Amsterdam, mainly under the influence of informal contacts with David Gregory and his circle. Physico-theology appeared not so much as a fresh start as a transformation of the older mathematical approach of philosophical and religious questions.