After the Muslim conquest of Jerusalem in 638, the Patriarch of Jerusalem invited Umar to pray inside the Church of the Holy Sephelcure. Umar thought rationaly, and realized this could endanger the sites Christian status which could cause major problems down the road. Instead of Umar declining to pray inside the church, he either didn't think or for some reason was convinced to pray inside it.
Because Umar had prayed there, the site would likely be turned into a mosque in a few decades. Would the conversion of the most holy site in christianity to a mosque cause a sizeable rebellion of the inhabitants of a mainly christian populated caliphate? Could the Byzantines use this as a rallying point for reconquests in the East?