The first assertion of the freedom of religion in Tanzania is the 1977 constitution after the unification of the states of Tanganyika and Zanzibar after their independence from the United Kingdom. Article 9, 13, 19, and 39.2 grants religious freedom, equality, and prohibit religious discrimination. The preamble and article 3 declare Tanzania to be secular.

https://www.constituteproject.org/constitution/Tanzania_2005

THE UNITED REPUBLIC OF TANZANIA Office of The Attorney General. “The Constitution of the United Republic of Tanzania of 1977.” Last modified 2005. Accessed July 5, 2022. https://oagmis.agctz.go.tz/portal/constitutions/eyJpdiI6IkFmY0NLQ1wvQWJsUmliZWUxbWh1MVpnPT0iLCJ2YWx1ZSI6IlIwdWx4VnQ3a3plWnFqb0pjcnlvQlE9PSIsIm1hYyI6ImEwNTQxYzdhNTIxMThlYWJlNzU3Mzc5NzZkMmRiYWJkODc4Nzc1ZjVjYjY0NTQ5OTQxYzg3NmYwMDZlN2Y5ZjkifQ==