The paper reports partial findings of a research project into Croatian ethnonationalism (Croatian: narodnjaštvo) as a religion (in the sense of a human invention of the sacred). The practical problems are as follows: ethnonationalism as a religion, which implies inter alia that an ethnic community (Croatian: narod) has the potential and/or capability to develop into, and ought to become, the substratum of a (nation-)state; consequences of ethnonationalism, which include the unattainability of ethnic democracy, ethnic economy and ethnic maturity; conditions of Croatian ethnonationalism, primarily the Catholic Church as a condition in 1961-71, and also before and after the period, especially since 1990. Theoretical problems, i.e. inadequacies in scholarly knowledge of the practical problems, include the following: firstly, Croatian Constitutional Court jurisprudence on ethnic and religious communities; secondly, systematic history of law and state in Croatia and Yugoslavia 1945-90; thirdly, transformation of both communism and catholicism into ethnonationalism; fourthly and fifthly, social structure and representation/agency. To attain the general goal of the research project, which is the use of reason in public affairs, the research is carried out within the theoretical and methodological framework of an integral theory of law and state which includes a modified Lasswell and McDougal's policy analysis expanded by historical institutionalism and critical theory. The subject-matter are the following features of Catholicism as an institutionalized religion, especially in Croatia 1961-71: (1) law, i.e. (1.1) sources of law; (1.2) internal law (organs, members, means); (1.3) external law (relations with the state and non-Catholics); (2) the Church and economy; (3) the Church and nation; (4) Catholicism on theory and practice. The hypotheses (which are ideal-types and as such cannot be either verified or falsified conclusively) are that ethnonationalism in Croatia is a consequence of, inter alia, the following beliefs maintained by the Catholic Church in Croatia in the 1960s and to a significant degree later on: 1. the only acceptable relationship between the Church and the state is the partnership of two legally equal public orders over the same subjects within which the Church has the exclusive power to regulate matrimonial and other family relations, and the power to control education in public schools; 2. peasant family is the basic organic human community; 3. the subjects to the ecclesiastical - originally feudal - power tied in fact to land make the ethnic community (Croatian: narod), which is united with the clergy into the Christian community (Croatian: kršćanski narod); 4. since fundamental truths are accessible by theology only, and practice is an application of theory, practical knowledge, especially on the appropriate relationship between the Church and the state, is valid only if in accord with Church teaching. The evidence presented in the paper supports to a significant degree the hypotheses. The research findings contribute to the solution of all the theoretical problems, providing major contributions to the second and the third: the most probable reason why the Catholic Church in Croatia was rather silent in the Yugoslav and Croatian Spring 1961-71 and quite vocal since the 1990 is the Croatian Church's allegiance in matters of Church and state more to the First than to the Second Vatican council (which abandoned the Church's "divine" right to be co-sovereign with the state, exposing the "right" as a human invention of the sacred); the Church's ethnonationalism, which facilitates the political partnership of the Church and the state and ensures the dominant position of the clergy within the Church, has coincided with the interest of Yugoslav communists to retain their might and power by a metamorphosis, with the Church's assistance honoured by a concordat, into Croatian ethnonationalists, who, as newly born capitalists, have appropriated the greater part of the former socialist property and continue appropriating the greater part of present public goods. [ABSTRACT FROM AUTHOR]