Prior work has shown party activists and religious divisions to be two of the leading causes of party polarization in American politics. Using the Convention Delegate Studies, we examine the interaction between these two culprits and their impact on party polarization. We leverage a novel measure of secularism in the latest wave of the Convention Delegate Studies to demonstrate that active secularism is distinct both conceptually and statistically from low religiosity. Furthermore, we show that both religiosity and secularism drive party activists to take more extreme policy positions, to identify themselves as more ideologically extreme, and to exhibit less support for compromise. As the Democratic and Republican Parties have become more secular and religious, respectively, these results suggest religious polarization may compound existing divisions between the two parties and exacerbate the partisan divide in American politics.