This is a summary of the main letter from Muslims to Christians. I highly recommend you read the whole thing at:
From 138 Prominant Muslims to Christians around the world:
Muslims and Christians together make up well over half of the world’s population. Without peace and justice between these two religious communities, there can be no meaningful peace in the world. The future of the world depends on peace between Muslims and Christians.
The basis for this peace and understanding already exists. It is part of the very foundational principles of both faiths: love of the One God, and love of the neighbour. These principles are found over and over again in the sacred texts of Islam and Christianity. The Unity of God, the necessity of love for Him, and the necessity of love of the neighbour is thus the common ground between Islam and Christianity. The following are only a few examples:
Of God’s Unity, God says in the Holy Qur’an: Say: He is God, the One! / God, the Self-Sufficient Besought of all! (Al-Ikhlas, 112:1-2). Of the necessity of love for God, God says in the Holy Qur’an: So invoke the Name of thy Lord and devote thyself to Him with a complete devotion (Al-Muzzammil, 73:8). Of the necessity of love for the neighbour, the Prophet Muhammad r said: “None of you has faith until you love for your neighbour what you love for yourself.”
In the New Testament, Jesus Christ u said: ‘Hear, O Israel, the Lord our God, the Lord is One. / And you shall love the Lord your God with all your heart, with all your soul, with all your mind, and with all your strength.’ This is the first commandment. / And the second, like it, is this: ‘You shall love your neighbour as yourself.’ There is no other commandment greater than these.” (Mark 12:29-31)
In the Holy Qur’an, God Most High enjoins Muslims to issue the following call to Christians (and Jews—the People of the Scripture):
Say: O People of the Scripture! Come to a common word between us and you: that we shall worship none but God, and that we shall ascribe no partner unto Him, and that none of us shall take others for lords beside God. And if they turn away, then say: Bear witness that we are they who have surrendered (unto Him). (Aal ‘Imran 3:64)
The words: we shall ascribe no partner unto Him relate to the Unity of God, and the words: worship none but God, relate to being totally devoted to God. Hence they all relate to the First and Greatest Commandment. According to one of the oldest and most authoritative commentaries on the Holy Qur’an the words: that none of us shall take others for lords beside God, mean ‘that none of us should obey the other in disobedience to what God has commanded’. This relates to the Second Commandment because justice and freedom of religion are a crucial part of love of the neighbour.
Thus in obedience to the Holy Qur’an, we as Muslims invite Christians to come together with us on the basis of what is common to us, which is also what is most essential to our faith and practice: the Two Commandments of love.
In the Name of God, the Compassionate, the Merciful,
And may peace and blessings be upon the Prophet Muhammad