Sava Nemanjic secured autocephaly for the Serbian church in 1219 and became its first archbishop, and in doing so he built something that outlasted every Serbian state that followed. The church he organised carried the language, the law codes, the royal genealogy and the memory of the medieval kingdom through nearly five centuries of Ottoman rule, when there was no Serbian state at all. That is the reason its role in Serbian national identity is structurally different from the role of, say, the Church of England in English identity: it was for a long period the only Serbian institution there was.
The memory that matters most is a defeat. The battle of Kosovo in 1389 killed both commanders and did not immediately end Serbian independence, but it was transmitted through liturgy, through the cult of Prince Lazar as a martyr and through epic poetry sung to the gusle, until it became the central story of the nation. Its liturgical form matters: this is a defeat commemorated as a deliberate choice of the heavenly kingdom over the earthly one.
Serbia is overwhelmingly Orthodox, with a Catholic minority of Croats and Hungarians in Vojvodina and a Muslim Bosniak population in the Sandzak. The slava, the family celebration of an inherited patron saint, is a distinctively Serbian institution and is inscribed on the UNESCO list of intangible cultural heritage.
Church influence recovered rapidly after 1990. Religious instruction returned to state schools in 2001 as an elective alongside civic education, a change argued over at the time and now routine.