Due to limited access to either of Jeffrey K. Hadden's or Karel Dobbelaere's books or articles online, below are a few ideas to help get you started. Secularization is understood to be a term describing a society that has lost its "close identification with religious values" ("Secularization"). The term describes a society in which many of its institutions have become nonreligious or secular. Secularization is particularly a result of modernization. As society becomes more and more industrialized, it also becomes more and more reliant on science and rational thinking to keep creating new inventions and new markets. Many theories have been coined to explain reasons behind secularization. Dobbelaere himself was particularly interested in offering one general theory that diagnosed the reason as to why society becomes secularized. He did so by compiling the already existing theories and "building bridges between them" to create one, strong rationale, which he attempts to do in his 1985 article titled "Secularization Theories and Sociological Paradigms," published in the 46th edition of the Sociology of Religion journal, which you should be able to access via your school library's databases.
Essentially, it has been asserted that societies become secularized once they develop subsystems to help them function. If one were to consider religion to be the major system upon which a society functions, such as was seen in past societies like the Puritans, then other institutions like government, politics, education, science, and family become that society's subsystems. Once a society becomes more and more reliant on these subsystems to function, religious values become less and less important for that society (Encyclopedia of Religion and Society, "Secularization"). Societies can become dependent on subsystems for different reasons, such as the need for money, power, and the desire to seek truth. These subsystems also lead to the development of separate values related to the subsystems, such as the value of success, separation of church and government, and the validity of scientific reasoning. The more societies become reliant on these subsystems, the more the societies also "reject religiously prescribed rules" ("Secularization"). Rejecting prescribed rules leads to certain social realities: (1) Education is made public rather than taught by the church; (2) the church is separated from the government; (3) ideas concerning birth control and abortion are rejected; (4) literature and arts contain less religious content; and (5) science is valued outside of religious perspective ("Secularization"). While some argue that secularization puts religion in its proper sphere, which is the private sphere rather than public, Dobbelaere himself disagrees with this argument, asserting instead that family life has also become secularized ("Secularization"). Dobbelaere also disagrees with any assertions that secularization is a natural evolutionary process of society. Instead, in his article "Testing Secularization Theory in Comparative Perspective," he asserts that secularization is a result of "conflicts over issues" between opposing ideologies, such as "religious, a-religious or anti-religious ideologies" (p. 138).