In an age when persecution for the sake of religion was common in the world all around him, we find the Messenger 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. 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 Aisha 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.
The holy verses say,
"The mercy of Allah is close by the gracious." (7:56)
When those who believe Our words come to you, say,
Peace be upon you. Your Lord has undertaken mercy upon Himself.
If any of you does wrong in ignorance,
then repents after it and reforms,
He is the Merciful, the Forgiving. (6:54)