In 614 Chosroes II destroyed Jerusalem, killed 66,509 Christians and captured the Patriarch of Jerusalem (then Zacharias), other Christians and the True Cross. Modestus had been on his way to raise Greek troops to oppose this and was surrounded by Persian troops, having a narrow escape. Modestus was then chosen to stand in for Zacharias as Patriarch. He buried the monks killed at the monastery of Saint Sabbas the Sanctified and rebuilt the Holy Sepulchre, the city's churches and monasteries with help from John the Merciful, Patriarch of Alexandria. He became patriarch in his own right after Zacharias died in Persia when Heraclius visited the city to restore the True Cross in March 630. On Modestus's death he was buried in the Church of the Eleona on the Mount of Olives.