Fr Justin (Popovic) of Chelije writes:

Holy martyr king Lazar Saint Sava’s ideal and plan for his whole nation was: ‘Give up everything for Christ, but Christ for nothing.’ No one has ever realized this ideal and plan to such a full extent as the holy and great martyr, Tsar Lazar. He brought it about for his whole nation when he decided in favor of the Heavenly Kingdom and offered up himself as a sacrifice on the field of Kosovo, together with the whole Serbian people. He did this from the purely evangelic reasons recorded in our folk epic:

‘The earthly kingdom lasts only for a brief time,
But the heavenly kingdom always and forever.’

“We die with Christ, to live forever”, he told his soldiers. That Kosovo’s declaration and testament is regarded as the covenant which the Serb people made with God – and sealed with martyrs’ blood. Since then all Serbs faithful to that Testament regard themselves as the people of God, Christ’s New Testament nation, heavenly Serbia, part of God’s New Israel.

Source: “The Mystery and the Meaning of the Battle of Kosovo”