Source: British Muslim Heritage Centre
The Prophet (SAW) made a promise to Christians. As Muslims we respect the precedent of the Prophet (SAW) and should therefore abide by promises he made to Christians in the name of all Muslims.
In 628AD, a delegation from St. Catherine’s Monastery visited Prophet Muhammad (SAW) seeking his protection. He responded by granting them a charter of rights.
(St. Catherine’s Monastery is located at the foot of Mt. Sinai and is the world’s oldest monastery. Its collection of Christian manuscripts is so vast that it is second only to the Vatican in this respect.)