This cross-sectional study involved 470 Polish, Catholic (90%), married (67%), or unmarried cohabiting (33%) individuals (237 women and 233 men) and is the first to examine whether and how two dimensions of individual religiousness (inclusion vs. exclusion of transcendence (IT) and symbolic vs. literal interpretation (SI)) as assessed by the Post-Critical Beliefs Scale are associated with the participants’ perceived stability of their marriages/intimate unions. As expected, greater affirmation of God’s existence as real (i.e., higher inclusion of transcendence) and greater literal interpretation of religious doctrines (i.e., lower symbolic interpretation) were each associated with greater perceived stability of marital/cohabiting unions. Also as expected, the former pathway was mediated by sanctification for both types of couples. However, the mediation pathway from the IT and from sanctification to relationship stability was markedly stronger for those with more literal views of religious doctrine only for married couples, and this complex moderator effect was not found for cohabiting couples. The findings imply that Polish individuals with a firmer belief that God exists and a more literal belief in Catholic teachings are more likely to view their intimate partnership as sanctified, which, in turn, is associated with a lower risk of union dissolution.