--- - |2 Abstract Political secularization theories have predicted religion's decline in public and political life, and desecularization theories have predicted the reverse trend. However, there is little agreement on the timing of either phenomenon or even their existence. Until now, deep empirical tests of any of these were hampered by lack of historical country‐level data on religious preferences of governments (previously used data sets go back only to 1990). However, the new Government Religious Preference data set (GRP) measures state religion from 2015 back to the 1800s. Using GRP data, this article offers the first long‐term quantitative measurement of political secularization and in doing so, weighs in on competing claims regarding its timing. This article finds strong support that political secularization happened gradually over the long 19th century, accelerated after World War II, and peaked in the 1970s or 1980s. In contrast, the article finds only tepid support for the existence of political desecularization overall. - 'Journal for the Scientific Study of Religion, Volume 58, Issue 3, Page 570-590, September 2019. '