Christian Peper in his Introduction to Toynbee’s correspondence with Columba Cary-Elwes:
Toynbee was profoundly affected by [...] the reply that the fourth-century pagan orator Quintus Aurelius Symmachus made to the Christians as he urged restoration of the statue of Victory to the Senate House, Non in uno itinere potest perveniri ad tam grande mysterium. Toynbee insisted that no revelation was unique; “if the revelation of the One True God is to be accessible to to all men, it has to be diffracted” [Peper gives the reference to the seventh volume of the Study].
Mysterium should be secretum; but perveniri is correct, although one might expect pervenire. Peper did not take the quotation in Latin out of the Study, but Toynbee gives it in English in the seventh volume.
In the letters, Toynbee tells Cary-Elwes (June 8 1964) that he has read “the Symmachus Saint Ambrose ‘brush’” – Symmachus’s petition to Valentinian II in which the quotation is contained, and Ambrose’s reply to Valentinian. There are further references to Symmachus on Christmas Eve 1946, April 13 1949 and November 26 1951. Toynbee had inscribed the quotation in Latin on his 1946 Christmas card to Cary-Elwes, which arrived at Ampleforth on December 17.
In the twelfth volume of the Study, he writes:
I am [...] a Symmachan [...].
The quotation appears in English at the end of the American lectures published as Christianity among the Religions of the World (1957) and is alluded to at the end of the published Gifford lectures, An Historian’s Approach to Religion (1956).
Christian B Peper, editor, An Historian’s Conscience, The Correspondence of Arnold J. Toynbee and Columba Cary-Elwes, Monk of Ampleforth, with a Foreword by Lawrence L Toynbee, OUP, by arrangement with Beacon Press, Boston, 1986, posthumous
A Study of History, Vol VII, OUP, 1954
A Study of History, Vol XII: Reconsiderations, OUP, 1961