Freedom of religion was first guaranteed in the Algerian Constitution of 1963. Article 4 of the document guarantees this right, while also stating that Islam is the state religion.

References:

“The Algerian Constitution.” The Middle East journal 17, no. 4 (1963): 446–450.

“Constitution of Algeria.” World Constitutions Illustrated, Heinonline. Accessed July 19, 2023. https://heinonline.org/HOL/P?h=hein.cow/zzdz0026&i=1