Christianity and Islam share a changeful history. Closeness and distance, rivalry and mutual recognition, war and peace: This is how they have considered each other and met one another over the centuries. Islam established a foothold in Europe long ago, but is it also comfortably at home here? One thinks for example of the coexistence under the dominance of the Muslim Moors in Spain – a brief flowering and peaceful interchange between Jews, Christians and Muslims that came to an end with the reconquest in 1492 when Spain was completely reconquered by the Christians. Nonetheless, Islam has left its traces in Europe and the fruits of the intense interchanges between Sufis, Kabbalists and Christian mystics still inspire us.
Today, so called Christian Europe finds itself under constant strain. One reason for this lies in the conflict caused by the fierce competition for material resources which is often attributed to the “foreigners”. So we are confronted with the need for peaceful coexistence and understanding, particularly with people of the Islam faith living in Europe or arriving here every day. Ignorance and unfamiliarity concerning the habits of everyday life and the religious beliefs of the “others” are respectively abundant, and besides, Christians tend to know very little about their own religious roots. So fathoming the depths of one's own religion as well as that of others will be of great benefit to all.
Christianity and Islam are counted among the monotheistic religions, together with Judaism, so they share a belief in one god, in which lies their kinship, but also their biggest problem: the rivalry: which is the true religion of the one true God?
Different fruits of one tree
On the face of religion, God appears as a lawgiver promising eternal bliss to those who believe in him and follow his given rules. Rules and doctrines – taken as a gospel truth – separate people of different faiths. Behind the so called Christian and Islam cultures there are indeed not only societies with different religious customs but also different lifestyles that are more or less patriarchal, with widely differing family obligations and much more. Moreover, Christians and Muslims often tend to view each other through superficial and prejudiced glasses. But stopping at this external level makes it impossible to reach common ground. To meet each other openly requires mutually objective knowledge. Whoever acquires this knowledge will find the love of God and humanity in all religions, not actually hidden, but just a bit out of sight. So Christians and Muslims are called to discover this love in their own hearts.
Assuming all religions derive from one divine sphere and comparing them with a tree that grows from heaven to earth we firstly see the different fruits. This one tree seems to bear such different fruits that one may doubt their common origin. These fruits may – besides their diverging shapes – testify to love or demarcation, peacefulness or violence, to a path to god or small-minded persistence in external differences. Each religion can be understood as a system of rules as well as an inner path. He who tries the fruits may discover that differently shaped fruits have similar flavours. Our cognitive instrument hereby is – in the words of the Christian mystic Master Eckhart – the divine spark within the heart,  which is able to realize the unity beyond all words. It is up to each one to view things in this inner light and to follow it. So the individual life practice of every human being influences the ideas that others have of their own religion or of being human – as well as of God. And this is true in a much broader way than one might suppose.
Inner paths in both religions
The common depths of Islam and Christianity lie in the divine seed from which they both have sprung. Both religions have always encompassed inner ways walked by the mystics and gnostics.  While mystics seek the divine beloved firstly with their hearts, gnostics attempt to transform through gnosis. Both know that it is possible to encounter God in their lives. They can also encounter each other on their path – in increasing comprehension and finally as friends and companions.
What do people who walk an inner path in Islam or Christianity have in common? Without generalizing, when a human being seeks, finds and follows the inner light, it cannot be otherwise, there is common ground.
Wherever God is not seen as a person, not as a lawgiver, not even solely as a creator, the view upon the divine mystery opens. God, independent of all worlds, is nonetheless within all human beings, all things and all activity; thus explained Ibn Arabi (1165-1240), the “greatest sheikh” of the Sufis. “We are closer to him than [his] jugular vein,” says the Quran.  And: “Wheresoever you turn, there is Allah’s Countenance.”  Although He is identical with the things, Ibn Arabi continues, the things – and humankind – are not identical with him.  As a material being with thoughts and emotions aiming solely at material things, man has not yet begun to realize the humanness intended by god. For he should become a perfect man, an insan al kamil. God has incorporated his traits in the 99 most beautiful names, and he who loves God travels to Him through His names. At the end of this journey, after the extinction of the I (fanâ’) and after being re-established in God (baqâ’), is there complete oneness, particularly as God is the only real existence? Let the following speak for itself. God says: “And when I love him, I am his hearing with which he hears, his sight with which he sees, his hand with which he seizes, and his foot with which he walks.” 
The Christian mystics have developed a similar view. Summarized, Master Eckhart (1260-1328) put it roughly like this: Behind the creator-God of the Bible there is the Godhead, the highest, uncreated being, the groundless ground, and the innermost of the human soul wherein the “divine spark” glows, is in union with it. God is the being of all creatures, but we humans have first yet to find access to this state of being. Jesus Christ is the example of the perfect man, and everybody is able to become a son of God through love. The path to this lies in love and understanding from the soul-ground. Hence, love enlightens the human being and saves the soul from death. Master Eckhart describes this state as follows: “The eye in which I see God is the same eye in which God sees me; my eye and God's eye are one eye, one seeing, one knowing, one loving.“
Ultimately, it comes to this: "Who loves, has no religion but God alone"
 One has to consider here that neither Christian nor Muslim communities are homogenous: just as each Christian country in Europe has its own way of life, so do the Muslim countries – quite different to the one-dimensional view that popular opinion can have us believe.
 In which the capability of knowledge is not our own, but this knowledge is a realm we can enter.
 In Islam, the latter are called ârifûn.
 Allah is the one who speaks here.
 Futuhat al Makkiyah III 384.18.
 Hadith Qudsi.
 Having read Averroes (Ibn Rushd) and Avicenna (Ibn Sina) as well as Moses Maimonides – there is an abundance of common ground amongst the Sufis, Kabbalists and Christian mystics.
 From the sermon: Who listens to me will not be ashamed. Source: Internet
 Rumi, exact source unknown.