In an age when persecution for the sake of religion was common in the world all around him, we find the Messenger (peace be upon him) giving the Christian monks of St. Catherine in Sinai a promise of protection and exemption from taxes. A Christian woman may marry a Muslim and retain her faith if she so desires. Islam has to come willingly from the heart or not at all.
The enemies of Islam tried more than once to assassinate the Messenger (peace be upon him). It was only fair that such assassins be killed, but when they were caught and brought before him, the Messenger made them promise not to make the attempt again, then set them free. He taught 'A'isha to pray, "My lord, thou art the pardoning. Thou lovest those who pardon, so pardon me.' For to pardon is an aspect of his mercy.
Source: Salamat, Abdul Munem, The Life of Muhammad (may the peace and blessing of Allah be upon him), Dar Al Huda Publishing and Distributing House, Riyadh, Saudi Arabia, 1998. P.774-775