Tunisia is overwhelmingly Sunni Muslim, around 99 percent per most estimates including Pew Research's Religious Composition of the World, 2010-2020, with a very small Jewish community, one of North Africa's oldest, concentrated partly on the island of Djerba, and a small Christian population of foreign residents and converts.
Carthage, founded by Phoenician settlers on Tunisia's coast, was a major center of Punic religion centered on the gods Baal Hammon and Tanit before Rome's destruction of the city in 146 BCE, and Roman-era Tunisia, renamed Africa Proconsularis, later became an important early Christian center, producing theologians including Tertullian and, at nearby Hippo Regius across the modern Algerian border, Augustine, whose thought profoundly shaped Western Christian theology.
Islam arrived with the Arab conquest in the seventh century, and Kairouan, founded in 670 CE, became one of Islam's most important early cities in North Africa, home to the Great Mosque of Kairouan, traditionally regarded as a preeminent center of Islamic learning in the Maghreb comparable in local prestige to Mecca and Medina for regional pilgrims. Djerba's El Ghriba Synagogue, one of the oldest synagogues in the world by local tradition, remains an active pilgrimage site for Jewish visitors from Tunisia and abroad despite the community's small remaining size.