“It is without doubt true that all men, of whatever race or degree of civilization, are essentially alike; they constitute what certain authorities in anthropology have fitly called "a spiritual unity." But for the individual who cannot expect to find within himself whatever is necessary to understand and interpret this unity, and especially for the observer who does not care even to detect and recognize the existence of such a unity, the difference between Orient and Occident is a puzzle—perpetually baffling and seemingly insoluble.”

Source: Autobiography of a Yogi, Chapter 42 - "Last Days With My Guru"

As quoted in Words of Ecstasy in Sufism (1985) by Carl W. Ernst, p. 45
Variant translation: Allah, Most High, is the very One Who Himself affirms His Unity by the tongue of whomsoever of His creatures He wishes. If He affirms His Unity in my tongue it is He Who does so, and it is His affair. Otherwise, my brother, I myself have nothing to do with affirming Allah's Unity.
As quoted in "Husayn ibn Mansur al-Hallaj" at Sidi Muhammad Press http://www.sufimaster.org/teachings/husayn.htm
Context: God, Most High, is the very one who Himself affirms His unity by the tongue of whatever of His creatures He wishes. If He Himself affirms His unity by my tongue, it is He and His affair. Otherwise, brother, I have nothing to do with affirming God's Unity.

The Sun My Heart (1996)
Context: There is no phenomenon in the universe that does not intimately concern us, from a pebble resting at the bottom of the ocean, to the movement of a galaxy millions of light years away. Walt Whitman said, "I believe a blade of grass is no less than the journey-work of the stars...." These words are not philosophy. They come from the depths of his soul. He also said, "I am large, I contain multitudes." This might be called a meditation on "interfacing endlessly interwoven." All phenomena are interdependent. When we think of a speck of dust, a flower, or a human being, our thinking cannot break loose from the idea of unity, of one, of calculation. We see a line drawn between one and many, one and not one. But if we truly realize the interdependent nature of the dust, the flower, and the human being, we see that unity cannot exist without diversity. Unity and diversity interpenetrate each other freely. Unity is diversity, and diversity is unity. This is the principle of interbeing.

The first known occurence of such an expression is as "Omnesque mutuam amplecteremur unitatem in necessariis, in non necessariis libertatem, in omnibus caritatem" in De Republica Ecclesiastica by Marco Antonio de Dominis, Pars I. London (1617), lib. 4 cap. 8 p. 676 (penultimate sentence) books.google http://books.google.de/books?id=QcVFAAAAcAAJ&pg=PA676, cf. liberlocorumcommunium http://liberlocorumcommunium.blogspot.com/2010/03/in-necessariis-unitas-in-non.html.
Misattributed

The Little Book of Enlightenment (c. 1364)

We must strive for unity at any price and with all sacrifices. But while we are uniting and organizing, we must rid ourselves of all foreign and antagonistic elements. What would one say of a general who in the enemy’s country sought to fill the ranks of his army with recruits from the ranks of the enemy? Would that not be the height of foolishness? Very well, to take into our army – which is an army for the class struggle and the class war – opponents, soldiers with aims and interests entirely opposite to our own, – that would be madness, that would be suicide.
No Compromise – No Political Trading (1899)

Mont Saint Michel and Chartres (1904)
Context: Experience proved that man's power of choice in action was very far from absolute, and logic seemed to require that every choice should have some predetermining cause which decided the will to act. Science affirmed that choice was not free,— could not be free,— without abandoning the unity of force and the foundation of law. Society insisted that its choice must be left free, whatever became of science or unity. Saint Thomas was required to illustrate the theory of liberum arbitrium by choosing a path through these difficulies, where path there was obviously none.

Revolution in Laos: Practice and Prospects (1981) (excerpts)

Source: The German Ideology (1845-1846), Vol. I, Part 4.