After ousting IS militants from towns near Mosul, Iraq, Christian militias have reported of the devastation the group left behind, including the destruction of precious icons and ancient texts in the 4th-century monastery of Mar Behnam.

According to the Jerusalem Post, Iraq's largest monastery, located in the Christian-dominated town of Bakhdida, just 20 miles southeast of Mosul, was turned into a headquarters for ISIS police - those responsible for handing out draconian punishments to those who didn't adhere to the group's particular brand of Islam.

After overtaking the monastery, militants used explosives to destroy part of the ancient building, removed the site's crosses, and turned the monk's bedrooms into jail cells. Carvings in Syriac, the language spoken by Jesus, were scraped off the walls, and any mention of Behnam - the son of a repentant Assyrian king who built the monastery after killing both his children because they converted to Christianity - was removed. Islamic State also graffiti covers the monastery's walls, including the group's motto: "Remaining and expanding."

"Their fundamental goal was to destroy Christian history and civilization in the Nineveh plains," Duraid Elias, commander of the Babylon Brigades, a Christian militia that helped retake the site, told Reuters during a visit on Monday.

Mar Behnam monastery was captured by ISIS in July of 2015 along with vast swathes of northern Iraq. At the time, the group issued an ultimatum to Christians: Convert to Islam, pay a hefty tax, or die. Those who lived in the ancient building were subsequently forced to leave and flee to nearby Christian villages.

"You have no place here anymore, you have to leave immediately," the militants were quoted as telling the monastery's residents by a member of the Syriac clergy.

Over the weekend, the Christian soldiers - all members of the Baghdad-backed Babylon Brigades - re-erected crosses and the recapture of Bar Behnam was celebrated widely on social media.

Elias, the commander, said his unit had fought alongside the Iraqi army to retake the monastery and the surrounding village. He emphasized that his men - all part of a population of Arab Christians across the Middle East - are determined to rebuild their cities.

"We are proving to the world that Christians are not weak. We are stronger than they imagined," said Elias.

He added that his men have destroyed several homes belonging to IS fighters - and they will keep going.

"There are others. We are going one by one: for every Christian house they blew up, we blow up a house next to it," he said from atop the monastery, pointing out one such pair of buildings.

"This is war. An eye for an eye and a tooth for a tooth."