The current Austrian state has maintained the Basic Law on the General Rights of Nationals of 1867, drafted during the Habsburg Empire. This makes Article 14 of the document the first assertion of freedom of religion in the country’s modern history. However, deeper legal foundations for this right can be found in the Patents of Tolerance of 1781/82.

The Constitution of Austria was ratified on October 1, 1920 and reinstated on May 1, 1945. Article 7 bans discrimination, including on the basis of religion.

References:

English original text of the Federal Constitutional Law of 1920 883 (2010), "First Principal Article: General Provisions ," Constitution of the Republic of Austria. - October 1, 1920 : 883-890

“Austria’s Religious Landscape.” Austria Embassy Washington. Accessed July 19, 2023. https://www.austria.org/religion#:~:text=EXPRESSIONS%20OF%20THE%20BASIC%20RIGHT%20OF%20RELIGIOUS%20FREEDOM&text=According%20to%20Austrian%20law%20(Law,choose%20his%20or%20her%20religion.