What is the oldest written source in this country that mentions this right?
Norway
In 2012, Norway officially and explicitly guaranteed the freedom of religion through a constitutional amendment securing a secular Norwegian state.
Sources: “Northwestern SSO.” n.d. Prd-Nusso.it.northwestern.edu. Accessed June 20, 2024. https://heinonline-org.turing.library.northwestern.edu/HOL/Page?collection=cow&handle=hein.cow/zzno0083&id=3&men_tab=srchresults.