Secularism is the separation of religion and politics. So religious institutes are not responsible for education, healthcare, religion and government bodies.
Secularization is the process of separating religion and state and how religion loses social and cultural significance
Secularization thesis is the prediction that eventually through the increasing modernization, there would be a continued decline in religion membership as a result of modernization and that religion will become obsolete, however, that is not true, it just causes plurality. Pluraity is a social situation where diverse groups live together in peave ad there is social interaction.